Because obviously people in their 20s have exactly the same experiences, the same outlook on life and the same characters as in their 50s.
Kirk isn't "people".
No, he's a fictitious character.
I think that is, 'he's a fictional character.'
(you don't have to say that you stand corrected & defeated by my blah, blah blah.)
The idea of Kirk as thrillseeker is something you can buy or not buy, but for me the only genuine evidence of him having a deathwish comes in the films, once he is dealing with his mortality in TFF and later.
I see that more as Shatner and his own input rather than Kirk.
I remember the old 70s joke, "but captain, if we go down there it is certain death " .... "it is ... okay, let's go!"
but that's a gross oversimplification of the character.
A lot of the danger he faces in the series is simply a matter of being true to his duties and following his ethical dictates (and in ideal dramatic terms, doing so when duties and ethical dictates come into conflict -- now THAT's 'the best of both worlds' in my book.)
I remember in the M&C novels how they would not let Spock & McCoy let go of harping on Kirk's exposing himself to the cloud creature in OBSESSION, but again that points up the issue of feeling it is his responsibility, again the ethical dictates aspect.
Kirk might well be a wild man on shore leave, which is broadly hinted at (emphasis on the 'broad's), but that is not necessarily being a thrillseeker or a deathlover, unless he is having sex with a bucktoothed tribble.