I would never EVER EVER even consider pursuing a girl that is 17 or younger. Even 18 is pushing it.
As well, I would never consider pursuing a girl that ACTS like she is 17 or younger.
As far as whether they are attractive (or hot) or not, I can see how certain girls (17 year olds, 16 year olds) may BECOME attrative (or hot) but I wouln't call them good looking. The years between 17 and 21 are just such huge years in terms of growing your body and mind that there is just too much of a gap there for me to be interested.
edit to add: quite frankly, I like older girls anyway