It might depend on your definition of "surprisingly." According to this source, about 10% of American households are millionaires and about 80% of those are first-generation millionaires (i.e. they didn't inherit the money).

Additionally, only about 1% of those millionaires are under 35. (This is a point that gets lost in the debate about inequality, in my view; most wealthy people are old for what I think are extremely obvious reasons.)

https://spendmenot.com/blog/what-percentage-of-americans-are...