Discount brokerages such as TD Ameritrade, E-Trade and Charles Schwab have reported surging trading volume in recent months, particularly among their younger clients who have been driven to finally get in on the action as the market surges past records and hot bets such as cryptocurrency trading promise fast money for risk takers. (See also: Over 1M Join Waitlist for Robinhood Crypto Trading.)

According to Bank of America Merrill Lynch, $32.3 billion has funneled into global stock mutual funds and exchange-traded funds (ETF) in one week through Wednesday, representing the biggest inflow for any comparable one-week stretch since 2002.

Millennials, who have proven reluctant to invest in the equity market despite its over-nine-year bull run, are increasingly drawn to some of the market's newest and most risky areas, including cryptocurrency and cannabis investments. This has led many in the tech-savvy cohort to open brokerage accounts for the first time. Ameritrade, one of the first platforms to give its retail clients access to bitcoin futures, posted new account openings at a record in the most recent quarter, driven by a 72% spike in new business from the younger customers.

YOLO Drives FOMO

Devin Ryan, a brokerage analyst and managing director at JMP Securities has attributed the pent-up activity to the phenomenon "fear of missing out," or FOMO, among young investors. "It's all correlated," said the analyst, noting that Millennials, who are eager to get in on gains before the bull market ends, are more versed and interested in crypto and cannabis investments, relative to older generations.

Market bears, forecasting an inevitable correction after the market's stellar run since 2009, highlight the surge in activity as indication of a bubble ready to burst. Further backing evidence of a spike in investor euphoria, data shows investors have poured nearly $258 million into 10-day-old ETFs that buy companies related to blockchain, the technology behind cryptocurrency.

It's important to note that the cash inflow hasn't been restricted to Millennials, with the three main U.S. stock indexes continuing to reach record highs, as a lower corporate tax rates breed investor optimism. (See also: Holding Cash Will 'Feel Pretty Stupid’: Ray Dalio.)