The best 17, 18 year olds would blow up any 9th grader, regardless of skill assuming the 9th grader is aged to his grade (14 turning 15 sometime in the calendar year after Jan 1st of 9th grade) the physical development in those years is enormous and it really shows up when there are rare events when a 15 year old gets on the field with 17, 18 year olds...some UA tryouts, the USA U-19 tryouts, etc. I'd say unless you are a goalie or a lightening quick FOGO, the physicality of every other position just smokes the young ones out. A 9th grader making an impact on a varsity team is explained mostly by those teams / leagues being weak. On LI there have been so few like Nicky Galasso, and I don't see any 8th or 9th grader that was freakishly advanced as he was in the recent year or two.