Changing Terms for Minors' Savings Accounts

We have a savings account for minors where we tell the customer that the service fee will be waived until the minor reaches eighteen years of age. We do not tell them when that service fee will be charged, i.e., the quarter the minor turns eighteen or the quarter after the minor turns eighteen. Should we clarify this language? If so, do we need to send a change in terms to alert the customer that the terms of the account are going to change, since the account may have been opened five years prior. Are minors allowed to open accounts without co-signers?