This came up in my teaching studies classroom. We were discussing the physical and mental limitations of our instruments. Bass guitar being so large it would be very difficult for small hands to hold. Motor and co ordination skills are very basic at this stage. Also concentration is a lot harder to hold the younger the child. Victor Wooten started at 2 and Stephen "Thundercat" Bruner started at 4. In both these cases there has been a strong musical family supporting them. Pino and Flea both started in late high school so starting young does not necessarily equate to playing better. My view is that in the family teaching at 2-4 is fine. A small scale bass/guitar will be needed until the hands are big enough. Much of my own teaching would be centered around short bursts of information, listening, dancing and singing. In a music tutor setting I would increase the age to 5 and do much the same. Listening, dancing, singing, small scale bass/guitar. Focusing on lots of little/quick activities and short lessons. What age did you start? What age would you be comfortable teaching?