CBS, known for its programming aimed at an older audience, hopes ThirdAge will offer a strong complement, providing information and community services to baby boomers, the company said.
Privately held ThirdAge offers a syndicated news service, research programs, and commerce services to adults 45 years old and older.
CBS also will receive warrants for an additional five percent stake in the company and will gain two seats on ThirdAge's board of directors.
"As we continue to extend the CBS brand on the Internet, ThirdAge marks yet another strategic investment in what is now the fastest-growing segment of the Web user population--adults aged 45 to 65," CBS chief executive Mel Karmazin said in a statement.
As the Internet slowly erodes the TV networks' viewer bases, broadcasters are trying to marry their brands with online sites, hoping to capture the departing viewers' interest on the Net.
"Of the networks, NBC in particular has staked out a claim to the Internet in regard to aggregating online communities," said Anya Sacharow, an analyst who follows consumer content strategies at Jupiter Communications. "CBS seems to be following by going after a community closely linked to the demographics of the network."
Just yesterday, NBC moved to invest another $25 million in Xoom.com, an online direct-marketing firm. Last month, NBC announced it would partner Xoom with Snap.com to create NBC Internet (NBCi). NBC said it may spin off its NBCi as a separate company to capture some of the high valuation given to publicly traded Net firms.
Snap.com is owned 60 percent by NBC and 40 percent by CNET. CNET is the publisher of News.com.
In addition to the investment from CBS, ThirdAge received another $35 million in investments from several other investors, including Merrill Lynch, American Century Investments, and Hollinger International.