It's not ad agency people who "diss" the over-55 listener. It is their clients who tell them who their research shows are the primary buyers and the ones that are most open to trying new brands or products.
In the case of alternative rock, there is a reason why such stations don't do well on agency buys and tend to have lots of pawn shops and bail bonds advertisers... and that is the data that a lot of the listeners are younger males who are single with no family, not children, etc. That places the format outside the scope of many buys.