The boys are divided up into 3 groups with regard to age - the "big'uns" the "middle 'uns" and the "litle 'uns". The "big' uns" are at the most pre-teens. None of the exact ages are given, but the oldest boys are probably middle school aged boys. They are still interested in pretend play and have bouts of semi-maturity, but none of them are older teens. They are just too immature. This is significant because Golding wanted to show that even young children have, at their core, an evil nature and that it is man's evil nature, that is inherent, that can do him in, not the corrupting effects of society. The young boys are innocent children at the beginning of the novel and this is significant because they need to be as far removed from civilization age-wise as possible to still be "innocent".
I am not sure what you mean about what this says about teenagers today, since the oldest boys are most likely not teenagers (they are 12-ish). I don't think this novel would work if the boys were teens - they need to be as far away from adulthood as possible, yet still be old enough to have some semblance of rationale because it is the older ones that get things organized and the younger ones, when they are not crying, just go along and hope the older boys will take care of them.
It is interesting that Simon in Lord of the Flies by William Golding does not particularly fit into any of the age groups. Not quite "a big'un," Simon often stays behind and helps the small boys find fruit. He is a loner and goes off by himself. That he is an individual apart from the group allows Simon's intuitive nature the space to ponder what he observes and comprehend the evil within the boys.
Piggy, too, does not fit into an age group well. With a fattened body and thinning hair and glasses, Piggy appears older than he is. Thus, he represents the voice of adult reason, but a reason that is somewhat ill-formed and at a disadvantage since it does not come from a mature, respected adult.