It can certainly vary considerably, though. We've seen roosters hold off crowing until they're eight or nine months old, and we've seen them start as early as two months. (Talk about cute!)
That said, if you have a crowing bird that is younger than three months old, don't be in a hurry to find a home for "him," however. Hens can crow, too, so you will want to wait until your bird is about three months old to be sure. At that time, you should be able to see the difference in feather shape between hen and rooster (see how to tell if your juvenile bird is a rooster by feather shape, in the related questions below for more information on this.) We hear from people every year who have given away their "roosters," only to later find out that "he" turned out to be a great layer!
Also, if you DO have a rooster, and crowing is going to be an issue for you or your neighbors, all is not lost! The No-Crow Rooster Collar is a safe and effective way to reduce both the frequency and the volume of your rooster's crows! You may just be able to keep your rooster, after all.