I can think of nothing I would hire a 14 yo girl for. There is nothing less reliable than a young teenage girl. At 16 or 17 they seem to have more of thier "stuff" together. That being said, if I needed someone to clean stalls and feed horses I wouldn't care about how much experience they had I would care about haw they would get to work. If they did a poor job or were absent or late a lot then they would no longer have a job. I wouldn't waste time interviewing someone to muck stalls particularly if I wasn't going to pay them.
In this case I think you would have to find the CORRECT teenager. I know plenty of teenagers who are respsonsible, and I know just as many who aren't. So I guess it really depends on the kid.