I remember reading in Hardy's Far From the Madding Crowd that Gabriel Oak was (quote from memory warning) "of that age when people begin to stop prefixing 'man' with 'young'" - and he's then revealed to be 29. Ever since, I have thought of 29 as the age when one stops being young. That means I have nine months of youth left.
young man is fine,but i work as a cart pusher at walmart(a job usualy reserved for those under age 18), and i get called "hey kid". when i tell my coworkers how old i am they are supprised(apparently i look quite young for my age).
(All) The one difference you will all notice is that between the ages of 50 and 55 (or thereabouts) you will lose the ability to change the focus of your eyes. Regarding your eye as a camera, you could say that the lens is still quite clear and forms sharp images, the film is more-or-less as good as it was, but the focussing mechanism will seize up and there is nothing whatsoever you can do about it except get reading glasses.