There's no way of enforcing it. Even if we were to put in something where you have to state your age to create an account, kids are just going to lie and say they're 18 or older.
They already have a 'not safe for work' button for turning off 'offensive' memes. Nobody uses it. Age rating would be meaningless. People would ignore them.
IMGFLIP NEEDS TO HAVE AGE LIMITS ON MEMES OR LIST PEOPLE UNDER THE AGE OF 18 IN ANOTHER COLOR. TOO MANY KIDS WHO DON'T GET ADULT TOPICS. SO MANY UNDER 18 LOOK AT NSFW MEMES