How we cite our quotes: All quotations are from Star Trek II: The Wrath of Khan.
Quote #4
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.
Bones implies that youth lies in taking action, and that as long as Kirk sits behind a desk, his mojo will be gone. He's not entirely wrong, but Kirk getting back his command means a chance to learn new things to help him adjust, instead of reliving the glories of younger, studlier days.
Quote #5
SULU: I'm delighted; any chance to go aboard the Enterprise.
KIRK: Well, I for one am glad to have you at the helm for three weeks. I don't think these kids can steer.
The implication here is that age brings skill and wisdom that those young whipper-snappers don't have.
Quote #6
SPOCK: If I may be so bold, it was a mistake for you to accept promotion. Commanding a starship is your first best destiny. Anything else is a waste of material.
This is another way of saying you're only as old as you feel, and that if Kirk thinks he's still got it in him to go cruising around the galaxy, who cares how old he is?