It's best to waiting until around 5. Your child must be able to sit and pay attention for half an hour and accept that she won't be making music right away. One concern is that starting formal lessons too early (when she's likely to get bored and frustrated) is a sure way to turn your child off music for good. If your child is ready earlier, you'll know it by her intense interest in Grandma's piano or Daddy's guitar.
It's also important to remember that different instruments are better studied at different stages (which means diving right into the violin may not be the best approach). The recorder is a great first instrument that often leads to other wind instruments like the flute and clarinet. Plus, kids can start it as soon as their fingers can cover the holes. Other wind and brass instruments should not be attempted before your child's permanent teeth come in because pressure is put on the teeth when they are played. Your child can learn piano as soon as she can reach the keys and has the strength and dexterity to push them down. Although some kids can handle a violin at a young age, most experts agree on waiting until around 6 to begin string instruments (most come in smaller sizes for smaller hands). --Deborah Skolnik