Re: Daddy Kirk (spoilers probably)
Sam was supposed to be older, even if the only visual evidence was the cheesy porno moustache in "Operation--Annihilate."
In TV you normally denote age and/or seniority by how much clothing...facial hair...that sort of thing. So my guess is that Sam is probably at least a few years older than James and would probably already be off at university or some other school by the time his brother entered Starfleet.
Human instinct is pretty strong. You can't expect us to change overnight.
-Captain Jonathan Archer, 2151