“I feel like my formal education did not educate me at all for investing or planning for my future,” said Mr. Stillman, who has a bachelor’s degree in anthropology from the University of Rochester.

Only about a third of millennials currently hold stocks. And more than 80 percent of young investors — a far higher share than among older age groups — describe their investing strategy as “conservative,” according to survey data released this summer by the Legg Mason asset management firm.

But the survey, conducted as major indexes were surging to new highs amid ostensibly low volatility, found that the group expected to eventually take on more risk than their elders. Many have piled into virtual currencies like Bitcoin.

Accustomed to a steadily rising stock market and the allure of astronomical gains in cryptocurrencies, millennials have a far different expectation of what typical returns look like compared with investors who have seen more cycles.

Millennials now expect an average annual return of 13.7 percent, compared to the 7.7 percent return that baby boomers expect, according to a survey last year from the AMG asset management company. Young investors are four times as likely as boomers to consider themselves to be highly knowledgeable.

But when the slump overtook the market on Friday, many were stunned.

“They’ve never seen a sell-off like this, and it’s especially scary because they don’t know who to ask for advice — they may not have a relationship with a financial adviser they can call or text to walk them back from the cliff,” said Jason Dorsey, president of The Center for Generational Kinetics, a research firm. “For many of them, it’s been a pretty rude awakening.”