Right, I can only give my own take on this, so bear with me.
I think most guys, probably some women as well but mainly guys, go through a "credible music" phase through college/uni and into the first few years of work. That was certainly the case with me, though the fact it coincided with indie in the UK being at a peak probably helped. Thus, would I have been a fan of the equivalent of Starship or Domino when I was younger? I'd like to believe I would be, because they're cracking songs, but maybe not.
When I was back in that era, 16-21, I was Radio 1/XFM all the way. When Radio 1 got rid of Mark & Lard from breakfast I thought it disgraceful and a betrayal, whereas now I would completely understand they were wrong for the slot and I was being an idiot. (though I still wouldn't be listening to Zoe Ball).
I slowly was won over again by pop music though, to the point that I'm a far bigger fan of new mainstream music than I was when I was a teenager. Funny how that works.
Anyway, I suspect my point out of this ramble is that by playing pure pop you're not always going to get the youngest audience. And I suspect Capital don't mind this. Under 25s aren't much use to advertisers because (as the stereotype goes) they haven't got any money yet. I suspect they'd be happy if the majority of the audience were over 25 - in steady work, possibly with kids who are a drain on finances, and possibly ready to graduate to Heart. The under 25s are served perfectly well by Radio 1, who can cater to all their whims.