The universal translator converts all systems of measurement to the listeners standard.
It's a question of youth. Humans beings, if McCoy is the yardstick, keep getting older and older and older and even more useless as they get older and older and older... Human beings might be able to conceivably live for a very long time but they become too fragile to live anything close to a life of adventure, and would probably die instantly if they tried.
Vulcans seem to have a lot more usable prime of life equivalent to a human 30 to 45 that lasts for 150 years or longer.
Vulcans live for a max of 300 years. Sarek was two hundred when he got his dementia and with the correct medical care would continue to live on for another century despite his brain being completely rotten.
T'Pol was almost 70 yet looked like a fit 26 year old human woman in ridiculous fancy dress... Which would have made her 170 in Enterprise Squarred where she was not fit at all.