The silver shield pikemen, Argyraspides, from the war of the successors. These men had followed Alexander the Great in his campaigns and continued fighting in the wars of his successors. Most were over sixty by the time of their last battle. They formed a unit that was undefeated in battle, the only reason they surrendered was when the rest of the army had been routed and their camp, which had their wives, children, and loot, were taken by an enemy army. Afterwards they were disbanded and sent two at a time on suicide missions that they never returned from.
King Masinissa of Numidia was reportedly still leading armies at his death about 90, true insanity for his time, although presumably not as a Frontline General (he would probably lead his armies from a horse or an elephant).
In the fifth year of his reign, Pharaoh Merneptah, Thirteenth Son and Successor of Ramesses II, led a victorious six-hour battle against the combined might of the Libyans and the Sea People at Perire. He would have been about seventy-five at the time.
Early modern
Francisco Pizarro conquered the Inca at 50, with a long career as a Spanish soldier behind him. One of his underlings, Francisco de Carvajal, would become the ringleader of a national-scale rebellion at 80.
At the Great Siege of Malta in 1565, the Knights of St. John were led by Grand Master La Valette, who was 71 at the time, while the Ottomans were led by Dragut Reis, who was 80, and their ground forces were led by Mustapha Pasha, who was 70. All led from the front, and La Valette and Dragut were both wounded in action.
Cristóbal de Mondragón, Alba's understudy and a fellow veteran of the Spanish armies, died of old age at 82 (other sources for his birth year claim 92) while in campaign, mere months after fighting and winning his last battle.
Jean Thurel served in the French Army from 1716 until his death in 1807. He was 107 when he died, and was consequently known as "the Oldest Soldier in Europe".
William Hiseland was the last survivor of the English Civil War, living to the ripe old age of 112. He fought in his last battle (during the War of the Spanish Succession) at the age of 89.
Samuel Whittlemore, the oldest man known to have participated in the American Revolutionary War. At the ripe old age of 78 he single-handedly ambushed a British brigade, shot and killed three redcoats, was shot in the face, bayoneted 13 times and Left for Dead in a pool of his own blood. He was found a short time later, still alive and trying to reload his musket. The doctors said he had zero chance of survival. Even so, he recovered and lived another 18 years.
The Old Guard of Napoléon Bonaparte's Imperial Guard. There were also a Young and a Middle Guard, but the Old one was the creme de la creme of the greater French empire. Composed entirely of veterans of Napoleon's earliest campaigns, with at least ten years of service, they were highly valued by Napoleon, and in return they were the most loyal to him. Since they were the only soldiers who could dare complain about their conditions in front of the emperor, they were known as "Grognards" (grumblers). Militarily, they served as a reserve thrown into battle at a critical moment to tip the scales, though more usually were kept in the back to avoid taking losses. When the Old Guard broke and routed at the final stage of Waterloo, the rest of the French army followed soon upon hearing of it, such was their Memetic Badass status.
General Winfield Scott first obtained a commission in the US Army in 1808, and remained in the Army for the next 53 years, serving through the War of 1812, various conflicts with Native American tribes, was one of the commanding generals in The Mexican-American War in the 1840s, and although by then he was elderly and in poor health, he was initially the overall commander for the Union in the American Civil War. Scott was one of the most sober and realistic voices in the earliest stages of the Civil War, warning that contrary to the overwhelming majority of public opinion and the demands of politicians, the war would probably not be over quickly (Scott predicted a minimum of two years of fighting), and that the Union would need a long-term strategic plan to subdue the Confederacy piece by piece despite most people believing that a single battle and a quick march to Richmond could crush the rebels. His plan for a blockade along the coast and to cut the Confederacy in two along the Mississippi River would eventually become Union policy after they learned the hard way that just trying to rapidly march on Richmond would not succeed, and the blockade in particular would crush the Southern economy during the course of the war. (While it's true that small and quick blockade runners could get through, the large ships needed to transport export goods that the Southern economy relied upon were forced to stay at home.
Gerd von Rundstedt joined Germany's elite military academy in 1892, before Hitler had even gone to pre-school. One of the oldest field marshals and a dedicated Prussian officer, he lead forces in France and ukraine well into his late sixties — making him the oldest soldier in a senior command position in World War II, and quite definitely the most competent man in his age bracket.
Field Marshal Count Radetzky, already veteran of the Napoleonic Wars, won his most renowned victories, at Novara and Venice, aged 82. He still lived for 9 more years and actively ruled Northern Italy as a Viceroy of the Austrian Emperor. His death had been due to injuries in an accident.
Field Marshall C.G.E. Mannerheim. He joined the Imperial Russian Cavalry in 1882note In context: out of the gallery of major power leaders in WWII, he got into military school before Hitler, F.D. Roosevelt, Tojo and De Gaulle were even born, fighting in the Russo-Japanese War, First World War and Russian Revolution. After the collapse of Imperial Russia, he retired 1917 in the age of 50 as General, returning to his native Finland. Little did he know his great years were only ahead. He was immediately assigned as the commander-in-chief of the Finnish White Army, leading them in victory in the Finnish Independence War. He was again nominated as Supreme Commander as Field Marshall, leading the country through the entire Second World War. He was 78 when he retired 1945, taking the office of President of Finland. Sadly, his health broke down and he passed away 1951.
War of the Triple Alliance featured Duke Of Caxias, supreme commander of the Triple Alliance in the later part of the war, who was nearly 70 and still leading his troops in the vanguard.
Modern
Due to the nature of the job, some soldiers are affectionately known as "Old Man" or something similar despite being several years away from middle-aged.
The entire Heer (Army) leadership of the Wehrmacht in World War II. The average Heer NCO (one per ten-to-twenty men) was forty-something or older. The average Heer general (one per 2k-4k men) was in his fifties. For contrast, while the average Soviet NCO was also in his forties or late 30s, Soviet Generals were all in their forties as well. This was thanks partly to the purges and prewar expansion of the Red Army, but largely to wartime promotions.
The French Army senior officer corps before and during the disastrous 1940 campaign was populated by old veterans. The commander in chief General Maurice Gamelin had been born in 1872, his successor General Maxime Weygand had been born in 1867 and lived to nearly 100, and Field Marshal Henri Philippe Pétain had been born in 1856, making him the oldest man to ever become head of state in French history. When Prime Minister Paul Reynaud asked their opinion of Charles de Gaulle (who was 50 back then), they called him "an unruly kid".
The last serving soldier to have seen active service in WW2 left the British Army in 1983, with general officer's rank.
General Lauri Sutela, Commander-in-Chief of the Finnish Armed Forces, retired likewise 1983. He had served as First Lieutenant in Engineer Corps in the Winter War 1939.
Dr. Earl Fox of St. Petersburg, Florida was confirmed as the last WWII veteran in the U.S. military, having served in the U.S. Navy and Coast Guard from 1941 until 1999.
Wolfgang Altenberg, who retired in 1989, served as Chief of Staff of the Bundeswehr from 1983 to 1986 and Chairman of the NATO Military Committee from 1986 to 1989. He started his military career as a Wehrmacht soldier during the Second World War.
All Marshals of the Soviet Union, including the ones promoted in the last years of the USSR, were veterans of the Second World War with the Marshals promoted in the final years of the Soviet Union having served as junior officers during the Great Patriotic War.
Major General Alfred K. Flowers holds the distinction as the longest-serving Airman in US Air Force history, and longest-serving African American in the US Department of Defense. He enlisted as a supply warehouseman in 1965, rose to the rank of Master Sergeant, received a commission as a Second Lieutenant, and continued to serve as a commissioned officer until he retired in 2012 after 46 years on active-duty (for comparison: most military personnel can opt for retirement after 20 years of service).
Admiral Hyman G. Rickover holds the distinction of being the longest serving U.S. sailor, along with the longest serving U.S. service member in history. A career submarine officer and veteran of World War I and World War II, immediately after that war he was chosen to find a military use for the embryonic nuclear power technology for something other than making things explode. He thus led the effort to design and build the first-ever nuclear-powered submarine, USS Nautilus and many others to follow, literally inventing new technologies and uses as he went. For decades he trained generations of a new breed of nuclear sailors and set enormously high standards for safety and craftsmanship in the new military nuclear power industry. As nuclear technology grew in importance, so did Rickover's position, and he eventually became the first sailor to be promoted to four-star Admiral in a non-command position.* His position was technically as an engineer, technical designer, instructor, and standards and safety overseer, all enormously important, but he did not exercise command over any ships or armed forces besides those in his immediate directorate and design bureaus. He finally retired in 1982 after 64 years of active duty service, just after his 82nd birthday.* Much of the reason he lasted so long was that his position as Navy Nuclear Reactors Director was invented just for him and was outside any regular military chain of command, making him unaccountable to anybody besides civilian leadership, who all almost universally loved him, leading to him holding the position for over half his career.
There are a lot of (admittedly anecdotal) stories from people who served in Iraq and Afghanistan, where patrols would encounter jihadis of mixed ages. Most of them would be younger guys with AKs who fought like fanatics but employed Hollywood Tactics and fell in droves, but occasionally there would be an older man among them with an old Enfield or Martini-Henry or even an older traditional, home-made Afghan jezail. These guys were the really dangerous ones.
, already veteran of the Napoleonic Wars, won his most renowned victories, at Novara and Venice, aged 82. He still lived for 9 more years and actively ruled Northern Italy as a Viceroy of the Austrian Emperor. His death had been due to injuries in an accident.