Best time to be an investor? Yes. Best time to invest? Meh.

This may be the best time in history to be an investor.

Never has it been so cheap to put money into the market, and it's about to get even cheaper following Vanguard's recent decision to end online commissions for most ETFs. Financial advice is easier to get, particularly for people with smaller account sizes. And advances in technology mean investors can keep tabs on their accounts simply by pulling their phones from their pockets.

That increasing ease plus the strengthening job market are helping to coax more Americans into the stock market. Slightly more than half of all U.S. families own stocks in some way, the highest rate since 2007, when the Great Recession was beginning. And stocks, with their long history of providing better long-term returns than bonds and other investments, are one of the most powerful tools to help people's savings grow.

The only downside in all of this is that it didn't happen sooner. Because while it may be a good time to be an investor, it's not necessarily the best time to be investing. After a long run of more than nine years of gains for stocks, more voices along Wall Street are saying the good times could end in the next few years.

If they're right, it will be up to investors not to turn all these newfound advantages and trading tools into implements of destruction. Even though it's easier —and less expensive — than ever to trade, sometimes the best thing to do when markets are falling is nothing.

Barry Bannister, head of institutional equity strategy at Stifel, says the bull run that began for the S&P 500 in March 2009 may end by the first quarter of 2020. Predicted cause of death: continued interest-rate increases by the Federal Reserve.

The Fed has already raised rates seven times since 2015 off their record lows, and it says two more increases may be coming this year. Higher rates have historically put the brakes on stocks and other risky investments, and Bannister says the Fed's pace means the federal funds rate may cross a key threshold next year.

With profit margins already high, Bannister says the next decade will likely have weaker returns for stocks than the previous one.

"I think we're looking at a rotating and trading market for 10 years," Bannister said. "So the ability to move fast and be flexible is probably at a premium."

That's good for investors who are able to take advantage of their new trading tools. Funds that cover everything from foreign bonds to low-volatility stocks to global technology companies have all been getting cheaper to own. Improved websites and lower commissions also make ETFs easier to trade. So anyone with the foresight to move from losers to winners can do so quickly and cheaply.

But it's also important to remember that many investors, for all their confidence, have a long history of selling and buying at inopportune times.

When stocks are soaring, everyone's happy to pile into the market on the expectation that more gains are to come. The biggest year for flows into stock mutual funds was in 2000, for example, according to the Investment Company Institute. That's when the stock market hit its peak and the dot-com bubble burst.

When stocks are falling, meanwhile, antsy investors sell their holdings. That may help avoid losses, but it creates a tough decision: when to get back into the market? Many investors who dumped stocks during the Great Recession missed out on the big gains that followed.

That's why investors, as a group, have largely fallen short of the returns of the funds they invest in.

Consider Fidelity's Contrafund, one of the largest mutual funds by assets. It returned 10.9 percent annually over the 10 years through June 30. But not all of its investors stayed committed to the fund through that stretch. After accounting for dollars coming in and out of the fund, Morningstar says the average investor got an 8.8 percent annual return from the fund.

So, the best approach may be to take advantage of all these opportunities in a moment of calm, before markets are falling sharply, to ensure that your portfolio is set up in a way that will leave you comfortable regardless of the market's direction. If you can't stomach the idea of the stocks in your portfolio dropping 10 percent, consider shifting more money into bonds and other less volatile investments.

Then, take comfort in knowing that this golden era of investing means you can act quickly during the next downturn, but you don't have to.