If I walk into the candy store and pay 75 cents for a pack of gum, I'm not asked to show identification or sign my name to a contract. I hand the storekeeper my money, they hand me the gum. Society has no problem with children participating in such transactions. If I don't pay, the shopkeeper simply doesn't give me the gum. Easy to handle.
Internet access is similarly easy.
I can misuse my account, or fail to pay for it. In either case, the ISP can cut off my access. Every ISP I've dealt with required payment in advance, so they're not out any money if they disconnect me for non-payment. I don't see how it's more difficult to disconnect me if I'm underage. If I can't legally be responsible for my actions because our contract is invalid, then I similarly can't hold them to the obligation to provide me with access.