i don't think there is such an age. i used to, but talk to some "old" people and ask them--it's generally about 20 years older than the person you ask lol
for me, i don't consider someone "old" till they hit their 80's or so. this is about the time when people seem to attend more funerals than birthdays for their friends. high school reunions are small enough to be held in someone's living room instead of the conference center. most of the friends still alive are widowed instead of divorced. basically, death is really a part of your life.
though i do remember when 30 was old

(funny that...the closer you get to an age, the younger it seems!)