I don't see the problem. First, can any of you differentiate between a 40 year old and a 50 year old person? Second, how many of you know what a 50 year old person will look in the 23rd century? Even over the last 30 years, 50 year old people have been looking younger. There's a reason they say "50 is the new 30 (or 40)".
We know that James Kirk in the 23rd century was griping about growing old in Wrath of Khan on his 52nd birthday. And he looks nothing in that movie like Cage Pike (William Shatner was himself 50 in that movie).
McCoy: Damn it, Jim, what the hell's the matter with you? Other people have birthdays. Why are we treating yours like a funeral?
Kirk: Bones, I don't want to be lectured.
McCoy: What the hell do you want? ...This is not about age, ...and you know it. This is about you flying a goddamn computer console when you wanna be out there hopping Galaxies.
KIRK: Spare me your notions of poetry, please. We all have our assigned duties.
McCOY: Bull. You're hiding ...hiding behind rules and regulations.
KIRK: Who am I hiding from?
McCOY: From yourself, Admiral!
KIRK: Don't mince words, Bones, What do you really think?
McCOY: Jim, I'm your doctor and I'm your friend. Get back your command. Get it back before you turn into part of this collection. Before you really do grow old.