Just because the Sorting is over doesn't mean you have to wait around for the next one. We're always accepting students of any year. We generally recommend new players go for third or fourth years. You're still young enough to have several years left to have fun at Hogwarts, yet you're old enough that you can take some elective classes and generally know a bit more about the world than an 11-year old. It also puts you in a good position to become a Prefect in your fifth year. We always like to fill out our Prefect ranks.
Also, you get to go on Hogsmeade weekends. And, coincidentally enough, the first Hogsmeade weekend is coming up on March 15th. Make a character in time and you can shop and sip butterbeer and enjoy the other delights that a small wizarding village has to offer.
Other characters besides students are also wanted and welcome, of course.