Well, they do make a point of having the Vulcan candidate mention that Wes doesn't look like he meets the age requirement (& he's not yet 16 at the time)If I remember rightly, they go out of their way to make clear that the other 3 are normal entrants and only Wesley is an early admission.
If we assume a realistic minimum serving age of 18, there's still a slim window to suggest that the other 3 might be still be under that but slightly older than him, & still in an early entrance program. I didn't see anything to mention the other 3's ages. So, it's a slim ledge on which to build a head canon, but it does kind of help the canon along some