Well, I was 30 when I came to York... Most of my friends, with a couple of notable exceptions, were also mature students...
We went to pubs. There's a pub for every taste in York, and a few to spare. Some reckon it's better than for the young 'uns, as there is less in the way of clubs - I wouldn't know, I've never liked clubbing.... Out of our class of about 30, (archaeology), I think 7 or 8 were mature (in age terms - there was the occasion three of them tried to wheel me down the road on a wheely bin...)
The campus is a 15-20 min walk (or much less on a bike) from the middle of town, so the whole of York is within reach, if you're based on campus, plenty of scope for going out.
Will she be getting uni accommodation at first? Several of us were placed in a flat owned by the Uni, rather than on campus, although our department wasn't campus based, so that might have been part of the reason. Even if she's on campus, there will be other mature students about - I don't think the college blocks are limited by subject. But I never lived on campus, so I can't really say.
If she's looking to live off campus in privately rented accommodation, there is loads - York is well supplied with terraced houses ideal for students to share (this assumes she has people to share with, of course). Southbank is very popular, and a nice area, as are the Fulford Road, Heslington Road and Lawrence street areas.
It may be that her interview day doesn't represent the true demographic on her course - anyone showing people round will be a volunteer, possibly on the promise of free biscuits, so all the mature ones might have been off in the library...
And 24 isn't that old (although I know it makes a difference). I mixed well enough with the kiddies
, at department social stuff, and at the same time made longer lasting friends among the other mature ones. And being a mature student is a great advantage - once you've lived a bit, you appreciate just how lucky you are to be doing something you've really chosen to do, as opposed to just going along with what you think people expect of you....
I love York - it's a nicely sized city, compact, but because of the tourists, it has loads of shops and stuff going on, festivals etc. But 15 mins on a bike in any direction and you're in the countryside.