The beginning age of a switch hitter may not seem that important. How can learning at 4 or 5 years old create a significant difference from learning at 9 or 10 years old or even14 or 15 years old?
To create muscle memory a person must repeat an action 10,000 times; this means a switch hitter must repeat the action 20,000 times. Muscle memory is much easier to capture and retain at a younger age. This is not to say there is a perfect age to start the switch-hitting process, it is just to illustrate that as a person ages it is harder to learn how to switch-hit.
My recommendation to any aspiring switch-hitter or any parent or coach that wants to teach a player to switch-hit is to start no later that 10 years old. The reason I say 10 is this is right when players start to play in more organized baseball games; if a switch-hitter can integrate repetition with game experience they possess the tools for success.