You can't. Unless Imgflip requests ID cards to verify, there's nothing to do. You could ask questions only people above 13 can answer, but then again, geeky underage kids could answer them. Not to mention some teens won't know the answer to them.
Imgflip could simply be like YouTube and ban accounts that are by underage people. But then kids would start lying about their age. Also, Imgflip doesn't care. They're like "We can't really do anything about the underage users" and ignore them. By the way, yes that was a real comment by a community mod and I'm not gonna say his/her name because I know some mods are a bit biased and would ban me because of me complaining about them.
Caption this Meme