I give customers a "service agreement", which basically tells the customer the price, frequency, payment terms, etc. The word "contract" seems to have a million interpretations on LS. I think some guys are looking for something that will guarantee the customer will pay for the entire year, even if they cancel after the first cut. I'm not sure of the legality of that, I'm sure it could be done, but not many people are going to agree to an arrangement of that kind.
I would have them sign some sort of service agreement. So what if you're not 18. I don't know how many customers will be thinking "Oh, I can screw this kid, he's not 18 yet".