Gen Y is more than 1 quarter of the US population and outnumber the baby-boomers. We're the children of the baby boomers, hence the alternative name for Gen Y: 'echo boomers'.
It is only the very tail-end of that group which are still in HS or below. You need the numbers for the kids born between (say) 1998 and 2015-minus whatever year they start preschool.
Also baby boomers may have retired but most don't need to be in a nursing home yet (which is the really expensive part).
At least in Australia, the really expensive 'contingent liability' (i.e. future tax burden on Gen Y) comes from health care expenditure, which tends to sky-rocket as one grows older. Might be different in the US given the more limited government expenditure on public health.
Gen Y is more than 1 quarter of the US population and outnumber the baby-boomers. We're the children of the baby boomers, hence the alternative name for Gen Y: 'echo boomers'.