I coach, I started at 17 not intentionally, I just had a few friend's who asked for some help with their horses.
I found that I enjoyed it, so looked into getting certified. Here you have to be 18, you MUST have insurance if you are to be a qualified coach and there is a hell of a long course to do that costs a small fortune before you get even your introductory level coaching certificate.
You would be mad to coach without insurance particularly if you are a) using your own horse b) teaching on your property and c)charging for your time. Even coaching raw beginners on a 10h shetland while leading them is dangerous, if the kid falls off and does lifelong damage to themselves, they you are in BIG sh!t. If you wind up being sued under 18, that's your whole life ruined depending on the severity of the suing. You may end up paying it off for the rest of your life so say goodbye to any other dreams you may have that involve money.
Back to my 'amen Spyder' comments - I can't handle kids teaching other kids when the kids that are doing the teaching barely know how to ride themselves. Unless you have had a bucket load of experience, can problem solve effectively, ride a client's horse is they are having problems and be able to 'fix' it, and get on your own horse and impress a potential client, I wouldn't bother. You're better off to get more experience with your own riding first, try to build up a name for yourself in the competition arena before trying to get clients to coach.