Yes, they happen. Ever heard of the Game Grumps? Well there is a guy in it named Arin Hanson, also known as Egoraptor, and he met his wife at an anime convention 12 years ago. He was 15, she was 13. And they've been together ever since. He is now 27, and she is 25. They got married in 2013. So yeah, it's possible. A bit rare to find something like that, but possible.
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The lovely couple, Arin Hanson and Suzy Berhow!