Most studios will take kids as soon as they're potty trained. For the first few years though it's more of a creative movement class than really ballet. Some studios have combo classes, half hour tap (mostly learning rhythm and right from left, basic steps) and half hour ballet (again, basic steps, right from left, general coordination). Some studios will add tumbling into that (forward rolls, backbends...)
For a full hour long ballet class I'd say around ages 5-7 depending on the child.