What age should children start learning to play the violin? 

There is no "wrong age" to start the violin, however, wherever possible I think it's great when kids start to learn at a very young age. I started at age 3, which means that violin just feels like a second language. That said, there's no real disadvantage to children starting later, especially if they're motivated to practice. Whatever age your child is now is the right age for them to start!