For younger writers, I highly recommend the Scholastic Write It! boards. This is the community that essentially raised me as an author. I'm still very close friends with a lot of the people on/alumni of the boards, though I no longer traverse the site. These boards are heavily moderated for safety purposes by a very nice mod named Bronwen. Give it a whirl!
The community I'm currently in is the Figment writing community. I'm not as invested in this community, but it's a good place for writers to network and post some of their work.
Finally, there's NaNoWriMo, which, besides being a community, is also a contest/objective. National Novel Writing Month in long hand, NaNoWriMo is a sponsored challenge in November to write a novel/reach a word goal in that one month. There are two websites, the adult and the youth. Both have vibrant communities, and either way the challenge is hugely fun and something to be proud of. I've won NaNoWriMo three times now, and I do recommend it.
So check 'em out, and have fun in your writing journey!
Pictures via Scholastic and NaNoWriMo.