This is obviously a question about wunderkind, and to some extent about child labor laws. The exemplar would be some hypercompetent young founder, but hitting it big in crypto would be fine, or any other way of getting there.

Self-made is here defined as meaning the money cannot be inherited: specifically, no more than $13m in 2023 US Dollars can come from investments or gifts from parents (somewhat arbitrary; derived from the current US gift tax exemption). Obviously rich families help in many intangible ways; for the purpose of this question that's fine. If the parent gave them $500m and they're now worth $1.5b, that does not satisfy the definition, although still an impressive feat.

As far as I can tell, Kylie Jenner is the closest so far, having in 2018 had a net worth of approx $1.1b in 2023 dollars at the age of 20.