I don't think age matters. I base my decisions on maturity, ability, experience etc..
As far as being liable goes, I think It depends on the state your in, if you have medicaid, insurance, and if your parents sign something saying you aren't liable for any mishaps. I think It would be perfectly fine hiring a teenager for certain jobs. When I was 16 I generally rode green horses, giving them more miles under the saddle, teaching them to neck rein, working with them on there fears, etc. I did everything except the first ride. Last Year when I was 17 My hand was broken while training a horse. I didn't go after the owner because I knew the risk when I got involved, but I had a good insurance policy and as soon as my hand healed I was back on green horses. So in my opinion it all depends on what the 16 year old is like. I use teens for the first ride, generally older trainers fear hitting the ground more, and younger trainers are cheaper and usually fearless. However, I don't think I would let a 16 year old train my show horses.