This generally happens to werewolves once they undergo the First Change. As werewolves have a strong Healing Factor, it's understandable that aging would be somewhat slowed. There's a vignette in one book meant to emphasize this - a werewolf who looks to be in her early 40s is called in for a polygraph test, and completely confuses the cops when she admits to being in her 70s and the test shows she's telling the truth.
The same goes for the Sin-Eaters. The fusion of the Geist and human soul causes Sin-Eaters to stay roughly in their prime in terms of energy and ability. They don't necessarily stop aging, but they avoid most of the problems of old age, such as arthritis and organ malfunction, often resulting in this.
In Warhammer 40,000, the Eldar and Dark Eldar have lifespans that can be thousands of years, but in their twilight years look to be middle-age by human standards. Eldrad Ulthran was at least 10 millennia old at the time of his death but looked to be in his mid-50s, and Azdrubael Vect claims to be old enough to remember the Fall of the Eldar (making him at least 2000 years or more older than Eldrad) but is still hale and hearty.
For the Dark Eldar, this is merely their physical form. Powerful Psykers and Daemons can see past this, and see the Dark Eldar for the twisted and horrifying beings they really are. Yes, even Lelith Hesperax has seen better days.
With the usage of rare rejuvenation devices, its possible for certain humans to look only middle aged while pushing 80 to 100.
Space Marines are in a whole another level. Looking just as young while being as old as three or five hundred thanks to their genetic modifications.
The Emperor of Mankind himself has the appearance of man in his mid-late forties, but is in actuality tens of thousands of years old, having been born in ancient Anatolia at the dawn of human civilization.
Due to the Dawn Blade's properties, Commander Farsight is suspected to be in his fourth century. His race doesn't live past 40.