You need to be 16 for your own login, and 18 to own a tree. Younger children are very much part of the family, though: they're on the tree as cards looked after by a parent or grandparent, with their own pictures and their own This is me voice recording. For example, you can look after your son's card until he turns 16, then he can have a login of his own and take it over.
Still stuck? Talk to a real person: send us a message, or email support@mywholefamilytree.com