C.T.
Its really up to you. Suzuki-style lessons can start as young as 2 or 3. Most other styles will start after 5. Progress is much slower when you start younger, of course, and not all kids will have the patience to sit and practice. I myself started violin at age 9, but I started my son on cello shortly before he turned 4. My son has always had both a strong interest in music and an unusual ability to sit still and concentrate for his age, otherwise I wouldn't have tried this early. Its a pretty big commitment as a parent to start early like this. I'm involved in my son's daily practice, and we drive 25 minutes to find a good teacher who specializes in young kids. Overall, I'm glad I've started my son young, but if you have any doubt about his patience or how much effort you are willing to put in, I'd recommend waiting a while.