!rant

Odd question, but it's something I should probably know. How early should I be looking/applying for summer internships in development? I'm still in high school, and need something useful to do next summer.

It's definitively OK to start your age (I did my first (mandatory) internship in IT support with 16).
Since you're an intern, don't expect too much - but it's good to get a feeling for working a job you might be interested in and roughly what you (don't) want to do.

@nin0x03 I meant how early before summer, which is when I want an internship, should I apply? but your answer was encouraging anyway thanks :D

@DavidINC Ah, I read that one wrong. m)
You can apply in principle anytime prior.
Depends also on the company where you want to intern, really. If it's a big company where internships - especially in summer when interning is popular - are rare and soon gone, the sooner, the better.
If it's a little or medium sized company three to six months prior are a good time. Earlier is imho better as they might not have dedicated programs for interns.