This ain’t the forties. I don’t think I’ve ever seen an Air Force Senior NCO younger than twenty eight years old.
Age does not necessary correlate to sexual maturity.
There are endocrine disorders which postpone or even obviate puberty. I think a hypothalmus damaged by neonatal asphyxiation is the most common (one in twenty five thousand people) and some of those never have a puberty at all.
If they meet height and weight minimums they would still be able to get in.
Okay, not a common situation but this does come up.