note that it said "the year of their 18th bday", meaning the bday could be anytime 1jan-31dec. assuming a june 1 graduation date and equal distribution, the average age at graduation would be about 17.4 [17 years + (5 months passed thru may / 12 months in a year). hopefully you do better than this at cls.
but more accurately, the general rule of thumb is that a child should be 5 years of age when they start kindergarten; though the cutoff date can range between states/munis.http://users.stargate.net/~cokids/kindergarten_cut-off_dates.htm
seems like sept 1st is a pretty common date. if these children are 5 years old at the start of K, then they are 6 at the start of 1st grade, and so on. they are 17 years old at the start of senior year. assuming equal distribution across the year and a june 1 graduation, a more appropriate number might have been 17.75 [17 + (9 mos. sept thru june / 12 months per year)] however, with dates more often coming later than 1sept than before it (including california and NY, 2 of the most populous states, which with late nov/early dec start dates would be right at 1june, meaning an unadjusted avg. at right around 17.5), it is likely the actual number is lower than that 21.75. i'd venture to put it around 17.6, perhaps even slightly lower.
i also think that our sample group is comprised of t14 acceptees, not your average student. these people (particularly at a numbers-heavy school like NU) are more likely to have skipped a grade, started early, or finished school a semester or two early than their not quite as gifted counterparts. is this enough, coupled with the later start dates, to push it down to 17.5 (or, later, 21.5)? who knows, but it's probably damned close, and maybe even below it.
normally i wouldn't waste the effort, but you deserve a giant @#!* you for being such an arrogant a-hole here and such a sarcastic prick in the thread where you bombed on burgh, who i know IRL and who is a great guy.