It depends on the child. When the interest/passion is there and it is fortified by the maturity needed to sit still and practice, it's time for lessons. There also needs to be a good bit of determination because learning guitar can be frustrating and painful, especially on soft little fingers. Many kids aren't seriously interested until they are about 12 or 13, but there are those who might have a sibling who already plays, so they are driven to learn at a much younger age.