If it's true that we should invest in what we know, I'm wondering what the mall is telling me lately. I don't head to the shopzilla all that often, but lately I can't help noticing that there's a major hole in the market for 30-something clothing, especially for men.

The "kids" have plenty to choose from. Abercrombie & Fitch (NYSE:ANF), Pacific Sunwear (NASDAQ:PSUN), American Eagle Outfitters (NASDAQ:AEOS), HotTopic (NASDAQ:HOTT), and Wet Seal all cater to a decidedly college-and-under crowd, to the point where I not only can't shop there, I feel creepy just walking in.

What's left for the pre-codger adults? Guess? (NYSE:GES) I've never been able to pull off the stubbly European pugilist look. Department stores? Eddie Bauer and L.L. Bean? I'm not ready to retire to plaid yet, either, thank you. By default, I end up at the Gap (NYSE:GPS), where I am completely unmotivated and only buy what's on deep markdown. That firm's crummy performance of late would seem to confirm that I'm not the only one who operates this way.

That's why, as a shopper and an investor, I'm really looking forward to what American Eagle is planning for later this year. The Martin + Osa concept, aimed at the 25-ish to 35-ish, should arrive in the fall with half a dozen stores. American Eagle has recently begun signing leases -- one of them not far from Fool HQ in Virginia -- and has made it clear that these aren't testers but a real rollout. Some of the original Abercrombie crew is on board to help out, and there's a target of 10 to 17 more stores for 2007.

Investors who aren't satisfied with the recent and quite decent growth at American Eagle might want to give this idea a good eyeball. To give you an idea about the potential, the company, as well as some analysts, estimate than this is eventually a $1 billion opportunity. Though that sum is large enough to represent 50% of this year's revenues, I wonder whether the opportunity isn't even larger. There's a demographic lump working through that age. And I have a hard time believing that the 15- to 20-somethings who grew up spending at Eagle et al. won't continue those habits into their 30s if they get the chance.

There's clearly a market here, since, by my tally, Abercrombie fumbled the larger opportunity with Ruehl. That concept sounded like it was supposed to address this market, but if you've ever been inside one, you know it's just a loud, scary, faux-brownstone version of Abercrombie, full of the same scruffy teenagers, just with fatter wallets.

By my calculations, American Eagle shares are worth more than $30 each without Martin + Osa. That means they don't provide the large margin of safety they had just weeks ago. But if the concept takes off, the stock could continue to outperform for years to come. Stay tuned.

Gap and Pacific Sunwear are Motley Fool Stock Advisor recommendation. Gap has also been selected by theMotley Fool Inside Valuenewsletter.

After dropping into hefty bargain territory, American Eagle Outfitters was Seth's pick for the Motley Fool Stocks 2006 investment guide. Investors interested in similar values can get the annual stock guide free, along with a hefty discount onMotley Fool Inside Value-- where Gap is a current recommendation. If you'd rather try before you buy, a guest pass is also available.

Seth Jayson just wants a place to shop that doesn't make him feel like a Geritol case. At the time of publication, he had shares of Guess? and American Eagle but no position in any other company mentioned. View his stock holdings and Fool profile here. Fool rules are here.