Van der Zwaan Dance Studio
My daughter started taking ballet at 10 or 11 at Van der Zwaan Dance Studio in north Berkeley (Shattuck and Cedar). The classes indicate the age range, and although she's usually been among the oldest in her classes, there's always been at least one or two students her age. She's made steady progress, and she's still taking classes there as a 14-year-old. http://www.zwaandance.com/
There are lots of classes for 4/5 year olds at Van der Zwann dance studio on Shattuck between Vine and Cedar. Very nice studio!