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.)
I would love to see some data around this. Because what I keep hearing (being in India) from the usual suspect sources is mostly gloom and doom[1]
[1] https://news.ycombinator.com/item?id=27205734