Jeff Taylor, who is 45, is obsessed with age 50. But it's not just his own half-century milestone he's thinking about—it's everyone's. Every day, some 10,000 people in the U.S. celebrate their 50th birthday. These baby boomers and their broader cohort are expected to live longer and more actively—traveling, working, hiking—than any previous generation.

Yet there are few Web sites catering to Boomers. To Taylor, the famed founder of jobs site Monster.com, that represents a huge business opportunity. "I think there's room for a big brand in this space," he says.

You have to be 50 or older to join Taylor's new Web site, called Eons.com. The site entertains and edifies its members with a panoply of brain teasers, a longevity calculator, special features to help users share their memories, and a database of 77 million obits. Taylor hopes the site will grow into a monster brand, to rival Monster.com's.

What's surprising is how few Web sites like Eons.com have sprung up to serve this market so far. One of the pioneers, MenAt50.com, debuted before many Boomers ever ventured online, and perished in the dot-com bust.

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