It's one of the most powerful ideas in modern personal finance: Stocks become safer investments with time. But the idea is deeply flawed for a variety of reasons. There's no safety extra layer of safety in owning stocks for 10, 15, 20, or 30 years. But even if it were true, take a look at this chart from Marketwatch of the Japanese stock market.
The Nikkei 225 had an incredible run, peaking on Dec. 29, 1989 at 38,957. It closed today at 9,207. That's 22 years, and the stock market is down 76% over that period. Sure, there have been rallies along the way, but stocks haven't come close to the 1989 peak. The message in the market for long-term savers: Stocks are risky.
This isn't the same as saying, don't invest in stocks. No, it's emphasizing the risks inherent in owning stocks.