Unfortunately, this is just statistics. There are cases that lie outside of that age range. I know of two cases, personally.

Things like stress, drugs, childbirth, significant life changes, etc can trigger psychosis and latent schizophrenia at any age, it's just statistically more likely to happen during adolescence and the period right after.

Another way to look at this is that adolescence is when someone experiences new stresses, significant life changes, drug experimentation, etc, which can be triggers for schizophrenia especially during age-correlated prodromal phases.