I am 29 so I always expect that I could hire someone older then me. It isn't an issue for me personally because at my very part time job I'm a supervisor and all my employees are actually older then I am. Every single sitter has always turned out to be younger then me. My preferred age is around mid 20's just because that is what we've had the most good experience with. My last 3 were 24 and I loved all of them, I went several this year due to bad circumstances that weren't anyone's fault, surgery, a move... I live in a college town and I do now refuse to hire college students just because we've gone through so many. They move, class schedule changes, it's finals week, on and on. I bet an older college student would be fine but I kept finding ones that were about 19-21 and they would be responsible for a while and then flake out on me. That isn't always the way it is, but just my experience I guess.
I've never had to post an ad, I've always responded to ads and found people that way. I lay out exactly what I'm looking for so I don't waste anyone's time. I guess if I had to call it something it would be mother's helper, my current person calls herself a nanny even though she does all the extra stuff for us.