Ooh, great question!
I think a series with an older cast could definitely turn away some readers, particularly teenaged ones, since they're at that age when they're beginning to figure out who they are as a person and consume media featuring people/characters doing the same thing. But having an older cast could also be an interesting hook, like in the movie Red (but even that has that young woman who is thrust into the old pros' world).
For me, how much I can relate to a character is based mostly on their personality.
I'm subbed to this one series, Easy (careful, some nsfw stuff and man x man themes), about a middle-aged teacher who is still trying to find himself. He isn't particularly handsome, or fit, or dashing (so like a lot of 40+ year old men xD), and I can see how people can pass over it. But I enjoy it because he's introverted, shy, and a little grumpy at times, all of which can describe me as well.
But definitely looks are a huge part of it. Like vampires can be hundreds of years old, but they're usually very beautiful/handsome/young looking, and those are the ones that sell. I think people tend to be more shallow than they like to admit.