Piano, cello, and violin are the typical instruments for kids to start on at the young ages, even as early as 3 years old. Other instruments such as trumpet, clarinet, flute and saxophone can be started at age 6 at the very earliest, but more typically at 7 or 8. Trombone, tenor saxophone, string bass, and other bigger instruments (requiring more wind or a bigger reach) can also start about then, around 8 years old. In public schools, most band or orchestra programs now start in the 5th grade. Forty years ago, most instrumental programs started a grade earlier, in the 4th grade.
"El Sistema" programs in the United States, modeled after the one in Venezuela (where Gustavo Dudamel, artistic director of the Los Angeles Philharmonic came from [see episode on 60 Minutes]) start children very early on these typical orchestral instruments. I actually teach for one of these programs in Baltimore. We have a Kindergarten child on baritone! This is highly unusual and he's a little guy too. He does get a steady sound, but he can't hold it very long. Still, he loves it, and that's the key. Starting early if the child doesn't like it is a bad mix. In fact, there are several factors that should be considered when considering when to start a young child on an instrument:
1) musical readiness - Can the child sing in tune and move rhythmically? Any instrument is, after all, only an extension of the child's own musicianship. Without this musicianship already in place, any music lessons will teach a child only to be a technician. That is, the child will be able only to pluck out notes and will not be able to be musical—certainly not initially, and perhaps not for a several years. Here's an analogy: This is not unlike a child who can sound out the letters in words but does not understand the meaning of what he or she is saying.
2) psychological readiness - Is the child ready—even eager—to take correction from a teacher regarding the right notes, rhythms, posture, hand positions, fingerings, technique, etc.?
3) physical readiness - Is the child big enough for the instrument? (Stringed instruments are sized for smaller children.) For wind instruments, is he or she able to blow enough air? Does the child have enough small motor coordination skills to have enough early success that can help maintain initial enthusiasm.)
4) intrinsic motivation - This is probably the most crucial factor. At any early age especially, a child needs to have the inner desire to take lessons. This factor can trump all the other factors combined. That is, if there's enough "want to," that's all you need. Everything else will work out in time.
Note: Having said all that, there is actually NO such thing as starting too early. What?! What about all those readiness factors? Well, the above information applies to "formal" music instruction. The importance of "informal" music instruction in early music development, much like language development, cannot be overstated and needs to start much earlier than 3 years old—even before birth! Children can start absorbing music and sound in the last trimester in utero. A child's musical potential is a product of nature and nurture. The role that nurture plays in the earliest months and years of life is undeniably important should we want children to grow up to enjoy and participate in music.