Do 13+ instances exist?

If you do make one be very careful. A lot of countries have laws about what people under 18 can do online. Running afoul of those laws could get someone in a lot of trouble. Besides lying about your age and making an account anyway is an internet rite of passage.

I’ll also note that in Australia under 16’s aren’t meant to be on social networks and that it may also happening in other places too

There are plenty like mine, might not be explicitly for under 18's but it is non-NSFW

likely any except for .world, nsfw lemmy, and ones that explicitly say “18+ only!”

I’ve seen 15 year olds here.

Considering that different states and countries have different laws, which range from 12 to 16, it would be hard to find servers willing to host anyone under those ages.

Since it is possible to host your own fediverse server, in some countries, it would be possible for the parents to setup a fediverse server that their child could use.

This would only be legal in countries where "parental consent" is required. It would be illegal in countries where there is an absolute ban on children using social media.