I graduated in a class of 80 in 2009. About 15 or so kids in my class, myself included, turned 19 during their senior year of high school. None of us were as old as Jax, the oldest was born in January of 1990, but turning 19 in high school isn't some crazy thing.
I would say it is much more typical for someone to turn 19 in high school than it is for someone to be 17 when they start college, which Jax would have been if he was a sophomore in college like you said he should be.