Whether your child takes a music appreciation class or starts actual lessons, she’ll benefit mightily from being exposed to music. Kids can start formal music training as early as age 3, when brain circuits for learning music mature. In fact, studies at the University of California suggest that taking music lessons at age 3 can increase your child’s brainpower. However, many piano teachers prefer that children wait until they are 5, when their hands are bigger and they’re more ready to sit still and concentrate. (See more on music and toddlers.)

There is however no clear answer to this question, it really depends on the individual student, and what instruments are chosen to play. Wind and Brass students of course usually wait to start playing much later than string and piano students. There is no “quarter” or “half size” trombone or clarinet for them to start on, so starting much before age 10 or 11 is usually a waste of time and energy. Students for winds need to have their permanent teeth well in place. Sometimes waiting until age 12 or 13 will lead to much more success with these instruments. At the same time waiting until age 13 to start the violin in most cases would be putting the child at a a disadvantage, and would set them behind most of their peers of the same age. A 13 year old violinist may be already quite accomplished, whereas a 13 year old trombone player is probably just learning to make their first sounds.

Link to article at Babycenter.com