Everybody I knew was either a research assistant or a teaching assistant and, so, in a sense, worked their why through the Ph.D. program. Plus, that experience was a big plus seeking a job either as a college professor or as a researcher afterwords. I’m thinking, were it even legal, what could you hire a 13-year old to do? For that matter, what could a 17-year old with a Ph.D. do? School isn’t or shouldn’t be an end in itself but a means by which those with the aptitude are able to lead happy and productive lives.
They could do a post-doc.
Graduating early and starting a serious job ( with its on the job training) at a younger age adds up to thousands, ( millions?) over a working lifetime.