No reason to be 16, when Jason could've been 18.
There would've been no significant change to the story if they made Jason 18 instead of 16.
How hard would it have been to add a line where Jason explains he was held back a year in middle school for grades... thus he could be an 18 year old heading into his senior year of high school (as the film is set over the summer). The film wouldn't have read any different. Leigh still would've been hanging around high school teenagers and trying to subconsciously relive her youth.
In fact, it probably would've been a better story anyway because Leigh could've convinced Jason to stay for a year, graduate high school, then head to Vermont with a degree.
And of course... the film could've avoided the whole scrutiny of entertaining pedophilia notions!
The more I think about it, the more I think the Liz Garcia had some agenda to push, because it doesn't make any sense for him to be 16; they didn't even try and make him 17.