I think this was asked by someone already, but I do wonder. I think it would be cool if my sister joined the Fediverse (but blocked NSFW content). She is a high school freshman, but I can’t have her join an instance where you must be an adult.
I do have friends on other social medias that are 13+ with their teen siblings making an account to follow them, but what are the rules here??
If it's just about following the rules as a matter of principle, I suggest not doing that. Nobody is checking, and saying your exact age on public social media is oversharing anyway.
If it's about content moderation being strict enough to satisfy some comfort level, I wouldn't rely on that, but I also think 13 is old enough to start learning there are shitty people online and how to deal with them, preferably with some adult support.
And that is what I would recommend against, even on a server that does not ban that age. If someone's (young) age is relevant to a discussion they wish to participate in, I would suggest a throwaway account.