Being a brand for a specific cohort is a slow-death strategy, not a growth strategy. I don't think many people invested in Yahoo would prefer that to a high-risk strategy with a plausible upside.
That age cohort doesn't have the same Internet penetration as 20-30 year olds today. Even if boomers are declining in numbers now due to death, you can get a larger percentage of boomers on the Internet, and they can make more use of it.
(The racial/ethnic minority markets, especially older people, are also dramatically under-saturated, so there's a huge amount of growth opportunity there.)
And at some point slow-death while maximizing profits from each user actually is the right strategy for a business. I suspect Netflix DVD-by-mail business is largely operating on that model now.
