Could be the ones who bought bitcoins at $3, or $30, or $300.

Those mostly weren't millennials, they were gen X libertarians. The ones that held long enough to make some real money, anyways.

I know one millennial who mined a bunch of BTC when it first came out, and then sold all of it when it first hit $20.

I also know another millennial who bought a bunch of eth at the IPO, and has held through everything.