The portion of kaizendad’s comment that I replied to specifically mentioned “output”. I interpret this to mean economic benefit or “value”.
The time students spend in school is time they are not working. Children generate almost zero economic benefit until they are out of school and the type of person kaizendad references is likely to spend a lot of time in school. Their economic contributions probably do not start until their late 20s.
I think a persons maximum economic output is probability closer to middle age than their early adult years. Perhaps kaizendad and I simply misalign on the definition of “young”.