When do you think you should allow a child to have an email address? A Fediverse account is basically an email account, except that the primary inbox is shared with a lot of people rather than only one person (and the same goes for any social media account). If you wouldn't allow someone to use an email address, you probably shouldn't allow them to use a Fediverse account either.

At least one person who seems interested in the health of children expressed that "delaying children’s access to smartphones until high school and social media platforms until 16" is a good idea. https://www.anxiousgeneration.com/ https://jonathanhaidt.com/social-media/ https://www.youtube.com/watch?v=V0MXgA2sSn8