Per Hillhouse et al, I am a proponent of the Peel Drop which is considered, by that school of thought, to be a fastball. Basically, if thrown with good 12-6 rotation a "fast ball" will drop. So, this can be taught at an early age.
No later than 11. It's a good one to learn makes change up and fastball more effective. It can keep hitters off balance. IMO movement is necessary in 12u.