Source: Mrs. Gemstone, Flickr.

In a recent ESPN interview, MLB Commissioner Bud Selig said baseball has "never been more popular," citing record attendance. While it's true the league has set all-time highs in ticket sales, this metric may be misleading. The sport's fan base is aging, and it's a trend that doesn't exactly encourage optimism. To borrow an analogy from my colleague Daniel Kline:  Applauding rising MLB attendance is along the same lines as cheering recent increases in vinyl album sales. There could be other reasons for the growth, and none have anything to do with the sport's long-term health.

Another reason for the growth
Since 2000, the league has had 12 of its 16 best seasons in terms of attendance. In 1980, an average of about 20,000 people went to a typical MLB game. Now the number's near 30,000, SB Nation reportsThat's an increase of 50% during a time when World Series television viewership has fallen by more than half. Regular-season ratings are in decline as well. And as a Harris poll data shows, the MLB is now the favorite sport of just 14% of Americans, down from 23% in the mid-'80s.

So why are more fans going to games? Forbes' Patrick Rishe explained the issue last summer, writing:: have to consider the role that variable and dynamic pricing have had on attendance. Both pricing systems ... allow teams to more efficiently price their tickets based on real-time demand conditions.... Part of the reason why ticket sales have improved in years past likely has less to do with growth in the sport's popularity and more to do with the league's teams collectively being more efficient with how they price their inventory of tickets.

According to the Chicago Tribune, 26 of the 30 MLB teams "use some kind of flexible pricing system." Variable prices are set in the offseason, and allow ticket prices to vary depending on game time and opponent quality. Dynamic prices, on the other hand, rise and fall in real time, depending on supply and demand. The Giants and Cardinals use the latter, while the Mariners employ a variable pricing strategy, FanGraphs reports.

The bigger issue
By making it easier to buy tickets, live attendance is up. That's Economics 101, though it doesn't indicate the sport's overall fan base is healthy. In fact, TV ratings and popularity polls say otherwise. But those issues aren't even the biggest problem baseball is facing. What is?

Aging. The average World Series viewer is now almost 55 years old, up from 49.9 in 2009, according to Nielsen. The company's latest Year in Sports Media Report reveals that less than a quarter of all baseball fans are below the age of 34, and a whopping 50% are at least 55. That's an older demographic split than every other major North American sport outside of golf.

Percentage of Fans
Age 55 or Older
Golf 63%
MLB 50%
Motor Sports 49%
NCAA Football 44%
NCAA Basketball 40%
NFL 37%
NHL 29%
Soccer 27%
NBA 25%

Source: Nielsen Year in Sports Media Report, 2013.

Why are fans getting older?
There are many answers to this question, but the best may be the simplest: Baseball doesn't captivate kids like other sports do. Professional football has the most popular fantasy game to reel in teens and young adults, soccer has its global advantage, and college sports have, well, thousands of screaming students as fans.

The MLB, on the other hand, is best known for its slow pace of play and old-school traditions -- two things that don't exactly exude youthful appeal. At the start of this season, for example, the average baseball game was over three hours, according to Business Insider. In 2003, the average game was only 2 hours and 46 minutes long.

A pitch clock and robotic umpires are a couple ways to improve game speed, but as of this writing, neither are being explored seriously by the league. Speaking from personal experience, the sport's new expanded replay system, introduced this year, only makes the problem worse.

The bottom line
It's worth noting that from a financial standpoint, the MLB is doing very well. Leaguewide revenues passed $8 billion in 2013, compared to just half that 10 years earlier. But as fellow Fool Eric Bleeker wrote earlier this year, those are heavily influenced by gargantuan, potentially overvalued local television rights deals. And although it might take a couple decades, this revenue stream could eventually shrink.

If that happens, the league will be forced to think about how it can reinvigorate its fan base. I just hope it's not too late by then. Sparkling attendance figures are a nice talking point, but they ultimately make it easy to ignore the one, simple truth: Baseball's popularity is in decline.