The best ages to begin lessons is are 9 & 10. At this point players have begun to develop both physically and mentally and have a greater capacity to retain and apply pitching and hitting concepts. This however isn't the rule. Some players can begin as young as 7 based on their level of athletic ability and confidence.
For players under the age of 13 parents can play a major role, especially if kids are still playing with mom or dad in the backyard. Our advice when taking a lesson is to sit close by, and learn the language and drill sequences of the coach who's instructing. Try helping your young player replicate it at home without being overbearing. Be careful however, not to interfere in the lesson. The most important aspect of individual instruction is the establishment of a trust based relationship between both the coach and player. If the dialogue between the player and coach is continually being interrupted, this relationship is much more difficutlt to establish. Try to ask questions at the end of a session.
This is always a difficult question to answer, because there are so many variables affecting success; confidence, athleticism, temperament, field coaches style. In general, the ages where we see the fastest physical results are ages 9 & 10. It depends upon the type of adjustment(s), ability to play with more confidence, and willingness of the player to work on the adjustments independently away from the batting cage/practice mound. Our aim with individual instruction is to help players create healthy training habits and let the hard work take care of itself.