My niece started taking ballet lessons when she turned three a few months ago. She is now on her second ballet school, as we felt obliged to remove her from the first one.
At our neighbourhood community mall, there was a small dance school with large windows for all to look in to see adorable girls dancing around in pink tutus, and those tutus immediately captured my niece's heart, and she started asking her father if she could "learn ballet" too.
We waited until she turned three, and we happily sent her off to ballet lessons. I personally don't believe in starting kids in any kind of extracurricular classes that young. But the thing with my little niece is that she's always been musical and dance prone ever since she was a toddler. I know, I know, every aunt says that her niece/nephew is a little Mozart or Anna Pavlova in the making, but trust me, my niece sings and dances all the time, to the point of annoyance actually.