Justin Bieber & Sofia Richie: She's Legal, Baby!

Justin Bieber isn't dating a kid anymore -- his new girl, Sofia Richie, is now the BIG 1-8.

Justin and Sofia got their juice on Tuesday in North Hollywood, the day before she officially became a woman ... in age, anyway. They hadn't been seen together for about a week while Justin was overseas performing at a British music festival.

Now they're not only reunited -- they're legal! You can stop cringing.