This will ultimately get thrown out. The legal standard has always been those under the age of 18 can't enter into legal contracts, and misrepresenting yourself as over 18 doesn't suddenly create a situation by which you can be opted in to liability for a contract you couldn't legally enter into. Why? Because the whole reason you can't enter into a contract under the age of 18 is because the doctrine is that you can't fully understand what you're signing up for. In that same vein, if you're under the age of 18, you don't understand the consequences of misrepresenting your age. It's plain common sense.
The only reason this case hasn't already been tossed is because the mother is ignoring the lawsuit, and only went so far as to send a "letter". The moment she hires a competent attorney, this case goes away, regardless of what one federal judge says. If there's one thing we've learned during "The Trump Years", it's that federal judges very frequently make decisions that go against the law and are ultimately overturned.