In the 1980s and 1990s, baby boomers reached financial maturity, spending hundreds of billions of dollars on housing, leisure, retirement savings plans, transportation and other categories.

This likely played a role in a 20-year upward move for the stock market.

Of course, now the oldest baby boomers -- born right after World War II -- are near retirement, and younger boomers are past their peak spending ages as well. It's time to shift gears and focus on the next massive demographic trend: The 'millennials' or 'echo boomers', mostly born in the 1980s and 1990s, which are set to overtake the economy.

How big is this group?

Demographers suggest that there are (or were) 77 million baby boomers.

The Millennials: 82 million.

Across the national media, you'll read many stories about how these millennials are flat broke and unable to move away from their parents as they are saddled with lots of student loan debt and limited job prospects. To be sure, this group is struggling right now. And some will tell you that this group will never have the financial prospects of their parents' generation. (Although according to a new report by my colleague Nathan Slaughter, editor of StreetAuthority's High-Yield Investing newsletter, there's trouble ahead for the boomers as well. Slaughter calls it 'The Greatest Retirement Betrayal In American History.' Click here to get the report.)

Millennials' worries today are reminiscent of boomers' troubles 40 years ago.

Back in the 1970s, fears of permanent economic stagnation led some demographers to suggest that the 'baby boom' was a financial bust. That view proved to be short-sighted.

In a similar vein, when I got out of graduate school in the early 1990s, the job market was locked shut, and a few years of temp jobs led me to question my future. An eventual economic boom helped me and my friends land on our feet and do our patriotic duty: Spend with abandon.

Today's millennials are poised for the same opportunity: The U.S. economy is so large and resilient that the next robust economic upturn is inevitable -- even if it seems to be taking its sweet time getting here.

So how can you profit from millennial investing?

We can already draw some early reads on their changing consumption patterns.

Housing

In contrast to their parents, millennials appear to want to maintain a smaller environmental footprint, which likely means smaller houses than the ones we grew up in. These days, it's a lot harder to find families with four or five kids, as was often the norm back in the 1970's. (I'm the youngest of four, and between my three siblings and myself, there are five kids. Average kids per household: 1.25).

Right now, homebuilders are benefiting from the sale of larger, more expensive homes, largely because baby boomers are still flush and millennials are still struggling. But look for the tide to turn.

As millennials approach their 30s, they are likely to focus on smaller, less expensive homes. As an investor, you should focus on home builders that cater to this trend.

Cars

There's bad news ahead for automakers: Millennials increasingly prefer to live without cars, with many of them choosing to live in urban environments.

Of course, many of them will move out to the suburbs once they start families, but studies show that Millennials simply don't have a fascination with cars that their parents had. As a result, look for fewer cars per household, and look for those cars to last a very long time, as the trade-up trend starts to fade.

For investors, keep an eye on the automakers that appear to be making clear headway with millennials.

The E-Tail Onslaught

In an extension of a trend that has been underway for more than a decade, look for more retail sales to take place online, and fewer in brick-and-mortar stores. Millennials are becoming more deeply immersed in online spheres, as evidenced by the fact that social media companies such as Facebook and LinkedIn continue to grow at a rapid pace.

'For the echo boom generation, ‘showrooming' -- looking at products on the shop floor, but turning to the Internet to make the purchase at the lowest price -- is a way of life. It is this demographic that already accounts for a significant proportion of retail sales,' noted the website ULI.com.

As an investor, you should be spending more time researching which online retailers are building a strong and loyal base of customers. Conversely, you should tread lightly with any legacy bricks-and-mortar retailer that has failed to embrace the world of digital shopping.

Ethnicity

The millennials are much more ethnically diverse than their parents, which is already impacting consumption trends. For example, Spanish-language media and entertainment have grown at a fast pace over the past decade, and even as many Hispanic Americans eventually transition into English-speaking households, they are still likely to be avid consumers of Hispanic food and culture.

The Next Boom

Perhaps the single greatest spending trend in this group will be in preparation for the next generation. In a survey conducted by the Pew Research Center, only 15% of millennials expressed a desire to have a high-paying career, while 52% of them expressed the desire to be good parents as their number one goal.

This suggests that this 82 million-strong demographic will be buying a lot of baby formula and diapers, and consuming a lot of educational programming on TV and online.

Then again, I recall being flat broke in my mid-20s and though it didn't make me want to be rich, I eventually cared a lot more about earning a good living and owning my own home than I did when I was in my early 20s. So maybe these millennials will become as spendthrift as their parents, even if they now profess otherwise.

The Investing Answer: The millennials (or echo boomers) will only slowly become a powerful economic demographic. That makes this a good time to study their consumption patterns and start identifying the companies that stand to most benefit from their coming wave of spending. The decade ahead should show signs of rising spending from this group, and by 2025, they will be approaching middle age, which often represents the peak in discretionary spending for most people.