All three major U.S. stock indices have recently hit new highs, the latest in a long series of gains stretching back since the early days of recovery from the financial crisis in 2009. Investors who stayed the course in those dark days have been amply rewarded.

But what's next? Can this aging bull market keep lumbering forward? Or is a day of reckoning overdue?

It should come as no surprise that experts have mixed views on what will come next, but all agree this bull market is graying at the temples.

"We are currently at 89 months for this bull market, as calculated from a March 2009 start date," says Dan Milan, managing partner of Cornerstone Financial Services in Birmingham, Michigan. "The only longer bull market began in October 1990 and ran 113-plus months. The average bull market historically runs 58.9 months.

Some experts see longevity itself as a warning sign. Others don't.

[See: 6 of the Most Overvalued Stocks on the Market .]

"Since the bottom of the market in the March of 2009, we've seen the S&P 500 price index increase over 200 percent over the past seven years,"says Ryan Kwiatkowski, director of marketing at Retirement Solutions in Naperville, Illinois. "That should signal a huge red flag,"

But Mark Carruthers, a planner with Genworth Financial Securities in Congers, New York, says stock prices are not out of line with standard measures like the ratio between prices and corporate earnings.

"Statistically, the U.S. equity markets are now fairly valued," he says. "The 25-year average price-earnings ratio on the S&P 500 (is) 15.9 times (expected) earnings. The market is currently at 16.6. Tough to tell what comes next. However, we're certainly not overvalued by historical standards."

Going forward, he says, the key will be what happens to corporate earnings .

The good news: there's little talk today of "new paradigms" or similar rationalizations, as there was to justify the dot-com bubble of the early 2000s. Most views today are rooted in history and numbers rather than wishful thinking.

Kwiatkowski focuses on the shape of the stock market graph and the market's resilience after recent jolts.

"From what happened since August of 2015, we believe we're currently in one of the longer-formed market tops going back in time," Kwiatkowski says, referring to two recoveries from big selloffs during the past year. "There is normally a surprise factor that typically will send these aging bull markets into a bear cycle, but China devaluing their currency and the U.K. voting to leave the EU haven't been enough yet."

Still, he thinks it's wise for investors to proceed with caution, trimming risk and not counting on outsized gains going forward.

"Regardless of what happens in the short term, we want to make sure that our clients continue to hit their long-term financial goals regardless of what stage of the cycle we are in," he says.

Andrew Thrasher, portfolio manager for Financial Enhancement Group in Indianapolis, says a key indicator of market trends is breadth, or the number of stocks rising versus falling, or setting new highs or lows. So far, he says, breadth measures have not set off alarms, as they did in May 2007, prior to the market downturn that began that October.

"In fact, the recent high in the S&P was confirmed by many various measures of breadth," he says.

[Read: Why Apple (AAPL) Needs to Double Its Dividend Immediately .]

While believing the bull market could be drawing its final breaths, Thrasher urges investors not to make any radical changes.

"It's far too easy for retail investors to allow their emotions to take over, which can have a large negative impact on their investment accounts and ultimately their ability for the assets to meet their future financial needs," Thrasher says.

For evidence of the folly in trying to time the market, one need only look at bonds , he says.

"Everyone thought rates were going higher in recent years and as a result lowered exposure to the interest rate-sensitive REIT market," Thrasher says. "Bad move. REITs now have a 5-year average (return) of 15.67 percent ... and have been a market leader."

Jordan Niefeld, an advisor with Raymond James in Aventura, Florida, says that under today's conditions, a bear market would likely turn out to be a dip in the long-term bull market. "On average these are 20 to 25 percent declines but take 11 months to get back to a new high," he says of that type of temporary setback.

He notes that many of the red flags that typically signal as looming downturn are just not present today, including heavy speculation drawing money into stocks. Instead, there have been large inflows into fixed-income investments, he says.

Also, there has not been a weakening in the number of stocks setting new highs, nor a flood of downward earnings revisions, or a speculative spike in credit spreads, which is differences in yield between bonds of similar maturity but different credit quality. Nor is there evidence of a rise in defensive leadership, when only the safest stocks perform well, he says.

Although corporate earnings have not been soaring, accounting for the rise is P/E ratio , companies have been good at managing cash, allowing them to shore up stock prices with share buybacks and dividend increases, says Matthew Essmann, managing partner of Cornerstone Financial Services Birmingham, Michigan.

While many bull markets have shaky underpinnings late in the game, this one has some solid support from low interest rates and low inflation and slow, steady economic and earnings growth, Milan says. Low interest rates make fixed income holdings less competitive with stocks, and low inflation shores up the value of corporate earnings. So it's not a red flag that P/E ratios are above long-term averages, Milan says.

For most investors, Niefeld says, the key question is, If not stocks, then what? There just aren't many appealing alternatives.

So he recommends no more than housekeeping, such as dumping holdings one is already unhappy with, and making sure the portfolio's asset mix is in line with the long-term goal.

"With really no place to invest for positive returns in the world, U.S. equities, although admitting to all of the concerns everyone has, are kind of the best place to be for now in my opinion," he says, adding that, "You may want to stick with large-cap stocks, and stay diversified."

Even if the bull market is on its last legs, the end of a bull market is not the end of the world, says Rick Bender, a financial advisor at Savant Capital Management in Rockford, Illinois.

"The good news is that history tells us the average bull market lasts 8.5 years with an accumulated return of 458 percent during that time," he says. "The average bear market lasts 1.3 years with an average loss of 41 percent."

[See: 7 Ways to Tell if a Stock Is a Good Price .]

A bear market is no reason to tear your hair out, Niefeld says. It's a buying opportunity.

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