According to the medication guide that is included with the medication and available on the FDA's website, "RITALIN should not be used in children less than 6 years old because it has not been studied in this age group."
I've heard of doctors prescribing ADD/ADHD medication to children under 6, and in my opinion it should be a last resort. I do think that ADHD is a real condition and medication can help a lot of kids, but I think it's better to try behavioral approaches for really young children. Kids change so quickly and can be just naturally excitable, so I think it's hard to make an accurate diagnosis when they're under 6. More importantly, I would worry about how medications might affect their development, since they're learning so quickly and their brains are still developing. Even if Ritalin isn't dangerous for them, that doesn't mean it would have no side effects. For example, I know it can stunt growth--I doubt that's dangerous, exactly, but you might want to avoid it for as long as you can.
Click Here to search for a treatment center in your state that is right for you.