They're not going to let it rest. The fandom for Who, like Trek and Star Wars, has traditionally been predominantly male. In the case of Who, especially when the show came back in 2005, older middle aged males. Then Tennant comes along, and soon there are the Tennant fan girls. And there's a *lot* of them. It's the first time I've ever seen a fandom once thought of as almost exclusively the domain for nerdy men get overrun by the ladies. Enough to make the BBC take notice.
Smith had a crap ton of fan girls here in the States. Seems like I recall seeing more fan girls when it came to Who, especially when they were showing the stuff on the big screen, than guys. Also, keep in mind, that most fans who discovered Who through the newer iteration are more familiar with the Doctor being younger. The only reason we had an older Doctor was because of Moffat and his nostalgia for Classic Who. Like it or not, I don't think we'll be seeing an older Doctor like Capaldi again.
For myself, I'm less concerned with the Doctor's age these days, than I am with the quality of stories.