The sentence below is right from the rule book governing New England boarding schools athletic age eligibility

"Age: NEPSAC regulations state that a student who is 19 years of age or younger on September 1st of a given school year is eligible to compete in interscholastic athletics."

So If you're wondering why the Laxachusetts players look a lot older, the rule above explains it. If you play at a prep school governed by NEPSAC you can enter your senior year as a 19 year old, turn 20 after September first, and you are eligible. All but four players on the 2018 Laxachusetts team fall into the above definition (they'll be 19/20 years old their |Senior year) according to one of the dads we spoke to down at Adrenaline. It is legal, but as Ty Xander tweeted out, there is a pretty big physical difference between those Laxachusetts teams and the Long Island teams they played, whose players will be 17/18 year old seniors, not 19/20 year old seniors playing under NEPSAC.