DD is 9 and we use ipads for 4-5 years now. She has her own one with she got with a huge list of restrictions which we adapt depending on age and usage.
Nothing is allowed at the table, that includes parent's smartphones unless we operate our sound system with it. That also includes going out.
She is not allowed to have her ipad overnight in her room, she can take it upstairs during the day for a year or so now, we check browsing history every now and then. She doesn't have fixed times but knows that it will be restricted if she doesn't do her allocated tasks like violin practice, school work etc beforehand.
For YouTube we had strict rules, she was first only allowed it with us in the room, then only if DH or I vetted the clip and she could store it in Favourites. Now she is allowed to use it more frequently but obviously understands more about the 95% of rubbish YouTube is made off.
Mac - she uses it for BBC iplayer and homework but only downstairs where it lives anyway.
IT can be a huge source of information, she used it for homework and discovered facts which we never heard off. There are also lots of good apps and websites teaching spelling, maths, good games teaching concentration and reaction skills. And there is the huge amount of rubbish so it is your job to teach your child.
The same with applications for Whatsapp etc, a young child has no need. We use Skype with grandparents or if DH or I are away on business trips. She uses emails to communicate with her grandparents.