Little1 already gave some detail on why there is essentially no agency business placed on radio.
The main reason agency clients tell their agency what demographics to buy is that the client knows who is the consumer, and who buys the most. Beers seldom advertise to women, even if women do drink beer. They drink, generally, less and are not commonly the decision maker in the category. If a beer advertises to women, the return on investment will be low, compared to targeting men, because fewer women make buying decisions and few buy for personal taste. In other words, it's not worth the effort.
Same goes for seniors. Even Boomers have buying patterns formed over many decades, so advertising must first overcome preferences, perceptions, stereotypes, etc. Then the ads must sell product. In most cases, the added amount of advertising needed to sell 55+ makes the profit per sale minimal or worse. There is no return on the investment, and it may also be that the campaign would have to be changed to resonate with older consumers... creating confusion in the marketplace.
Ad agency clients from P&G to Budweiser spend tens of millions a year to research their consumers... proprietary research only seen by brand managers and such... and if any marketer were to discover an opportunity in 55+, they would jump at it. For the moment, it's just not profitable.