when is the best earliest age for kids to learn or attend formal music lessons?

I have always thought that the earlier the better. Now that I have a daughter, it doesn't work that way anymore. I realized that it's her readiness and interest to do things that matter. If she likes to do it and she's ready for it, then I don't even have to force her. Know the child's interest and readiness. If at 2 she's interested and ready to play the violin, then why not? :)