Internet meme that came about after some kid said it on a comment when Youtube.com was raided with porn. Usually said after something that adults talk about/kids wouldn't get. Not to be said in real life, like most other memes.
Anon A: I was wondering, do you guys prefer anal sex or vaginal sex?
Anon B: I'm 12 years old and what is this?
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Anon A: Do you think its worth it to have sex without a condom?
Anon B: I'm 12, what is this??
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Anon A: My girlfriend just blew me, shit was SO cash.
Another less funnier variation of the meme I'm 12 years old and what is this?
Said in forums that restricts the registration of people below 13. If you put the original version of the meme, which is: I'm 12 years old and what is this? Mods might think that you're ACTUALLY 12 years old and ban you because they're dumbass pricks, or just have a reason to ban you for the lulz.
Person 1: "I'm 12 years old and what is this?"
Dumbass Mod: "You're 12 years old? This forum is for ages 13 and up! *Bans Person 1*"