How does the Enterprise get to the galactic core in "The Magicks of Megas-Tu"?
In "The Lorelei Signal", Kirk, McCoy, and Spock are all aging at the same increased rate. Spock says that the rate is ten years a day, and they all grow super-old together. But Vulcans live for a little over 200 years, and McCoy is older than Kirk anyways. How are they all the same age?
Spock does say that he will retain his strength longer than the others, for that reason. But yeah, he gets just as wrinkly as they do.
Supposedly this is the draining of life force, not actual physiological aging. So you drop from 100% to 0% at the same rate. Except, of course, that Spock doesn't. The short answer, I guess, is that it's a mistake to expect realistic geriatric portrayals in a situation where Space Patrolmen are having their mojo drained by vampire space vixens. MST3K Mantra ahoy!
It isn't clear exactly how history got fouled up in "Yesteryear." It apparently has something to do with the impossibility of Spock being in two places at once, but then the problem ends up getting fixed by Spock being in two places at once.
The part that I've never really grasped is this. Kirk and Spock are off in exploring "Orion, at the dawn of its civilization." The implication is that everything goes wrong because there are two versions of adult Spock in the past. Now, since this has never kicked in any of the other times Spock has time travelled, it must be because both Spocks are present at the same time. But the dawn of Orion's civilization sounds like it's in the distant past, whereas he events on Vulcan would have just taken place a few decades earlier.
IIRC, the issue was caused by the researchers viewing a period of Vulcan history that Spock was supposed to be present for, but was instead in Orion. Since he was elsewhen, he wasn't able to be there to effect the changes that he needed to, so the timeline got frakked.