Here is a scary thought. Our three year old granddaughter has figured out how to use her mother's smart phone. She's fairly adept at the computer, at least when she recognizes icons. To me that is much too young, even though it wasn't intentionally taught. Children are a lot smarter a lot younger than we think. This means we have to plan in advance about technology limits.
Decide Appropriate Age : Despite obvious talent and desire three is much to young to play with a computer or a smart phone. She can watch programs we choose on them, but three is too young to have one of her own.
Each child will be different so giving a solid age would be a disadvantage to some children. Parents know and understand their own offspring and can set that limit as they see fit.
Decide Appropriate Use : At three, sitting on one of our laps while we use the computer is appropriate for our grand baby when it comes to the phones and computers. When she gets into school she may need to use something with internet access. That doesn't mean the child gets across the board usage. You decide if texting or e-mailing school friends is appropriate.
Set Limits : It's easy to get sucked into a game, conversation or research and have five hours pass in what feels like five minutes. This is as bad for our children as it is for us. Let them know how long they have to use whichever device employed, give a five minute warning and then it goes off.
Follow Them : This is the most important part of setting limits. We have to follow our own advice. Our children will see what we do and follow our lead. When our time is up, we need to turn it off or put it away also.