Are you talking about a contractual agreement? Although most companies will not allow a minor to sign a contract for services, it is not illegal to do so. The problem would come in if the minor wanted out of the contract because legally a minor cannot posses contractual capacity, which is a requirement for the contract to be binding. The minor could simply disaffirm the contract and he would be let out if it. An exception does exist for contracts made for necessities such as shelter. A minor could possibly be held to a contract which provides for necessities.
