Age 18 is a weird one. If you’ve reached the age of 18 and are unable to handle the content of Imgflip, chances are you’re unable to handle the content of life.
As for age 13: without any means of verifying ages, the under-13 crowd is just going to log into the 13+ website anyway. And, even more disturbingly, there’s nothing stopping child predators from creating accounts on the under-13 site and be presumed to be under-13 as well, with no or minimal adult social supervision. It’d be a catfisher’s playground. I don’t know why any tech platform is seriously considering it.
Trying to partition Imgflip along age-related lines would be a disaster for many reasons, but that’s the main one.