Hello Laura Price,
Welcome to Microsoft Q&A forum.
For the first question, you can change the birthdate of your account, but you have to sign in with the exact account. Also, the change might not be reflected on age dependent features, or you will need to verify your age. Just want to confirm first, what is blocking you to do so, and by "My son had used his as a teen for xbox", do you mean that he is using your account for Xbox or using his account under your family?
For the second question, being in a family group doesn't mean your files will be shared to everyone in the group. Your files are still in your control and you can decide whether to share them or not.
I'm waiting for your reply.