Average everything out and you'll get a number close to 18 which is the typical adult age in most countries.

>Sexual development, ability to bear children? Mid teens for most, earlier for some.

Let's say 15

>Completed puberty, full height? Mid to late teens.

Let's say 18

>Finished the fastest phase of brain development? Mid-20s.

Let's say 25

The average of 15, 18, and 25 is 19.3