By definition U23 is supposed to be for University Aged folks around the world. Covid, deferred enrollment (Greyshirt), Redshirts, Olympic redshirts, as well as holding kids back a grade (or two) in middle school have made it so our system is failing us with that regard.
Take Bo Bassett for example. He will turn 18 by the end of this calendar year. That means he is an old 17 year old HS Soph. He will turn 18 before halfway through his Junior year in HS. He will turn 19 before halfway through his Senior year. The same can be said for Jax Forrest.
At the other end - Spencer Moore was 17 years old as a True Frosh at North Carolina.
Speaking of North Carolina, the state itself used to have a rule against people who are sort of aging out even being able to compete their senior seasons. If you were 19 before the school year had started, you couldn't compete (something along those lines) in sports that year. There are other states that also follow that sort of scheme.
Andonian was simply a kid that went through school at the proper age(s) without being held back.