how to successfully report an underage tiktok user?

2018 was eight years ago… they would have to have been 4 or 5 then to be under 13 now. you said yourself there’s no proof, and until there is, tiktok is unlikely to do anything except maybe send an age verification notice.

could be a kid on their parents’ account, could be someone who just likes squishies, the comments and videos are no way to tell unless you’ve actually seen them and they look like a child or explicitly share their age.