DS started violin just before 3 after asking for lessons for nearly 12 months. He would have found piano difficult due to hand size. We decided against Suzuki and went with a teacher that usually teaches older kids. At 4 he reads music, tells you if his violin needs tuning (and is always right) and can tell what the string notes are by ear. He loves it. He practices ("plays concerts") most days, only 5-10 mins - it is just part of his routine. We were advised by a music teacher to start formal lessons when your child can read (ie usually 6-7). Still worked for us since DS was reading at 2. The best thing has been that he doesn't get songs 'right' the first time and it takes (a little) effort.
