when will ypu start letting your child use electric gadgets ?

My children started at age 3 y/o...but with strict time limit and supervision. We don't have a TV at home, so they watch educational videos online, and playing phonics and maths games online also have been helping with their learning greatly. Since they have learn the rules and regulations for using gadgets since young age, it is easier to control the usage (not addicted till cannot be seperated from gadgets,stuff like that).