Doctor Who is 50; Dr Who is 900 years old. Something like that. That time gets very mucked about in Doctor Who is rather the point, isn't it? But it still does weird things to the inside of your head if you watched it as a kid. The Doctor keeps changing and seems to get younger by the reincarnation while you of course are getting older, but you revert to being that kid when you watch it. It's the only way to watch it because it has child-like charm, in a very English way.
I don't know how old Doctor Who is now supposed to be, in Earth terms, and you are not supposed to be worrying about such things when on the grand escape from the humdrum that Doctor Who is supposed to be.
His newest sidekick, Clara, (who has kicked around before, as a sort of teaser, presumably for her co-starring role) does seem awfully young but that is the point of sidekicks. She's a girl who yearned for adventure and travel and here she is stuck being a sort of nanny to a family who have lost their mum. She's a good stick, then, a stickler for doing the right thing, not one of those selfish young things who only care about handbags and shoes and going drinking.
She doesn't seem to have a boyfriend. Who needs a boyfriend when Doctor Who is about to sweep into your life in his "snogbox", which is what Clara calls the Tardis?
She is also an enigma. There is much speculation, among those who speculate on such things (I am not one of them) about what she might be. She can't be just some girl; this is Doctor Who.