My son is 7 and we gave him a phone we won from a raffle. He mainly used it before to watch videos in YouTube or play games we downloaded from play store. A couple of months ago, I had him his own facebook account so we can message and installed a sim card so I can call when he has no internet. I can't tell when is the right age to have a phone. But, what I can say is, it is up to us parents to make sure that whatever age they get their own phone that they'd be responsible enough to use it.
I forgot to save my phone number on my son's phone so when I called him one time, he didn't answer. When I messaged him, he said sorry for not answering because he thought it was a bad guy. Maybe I instilled the right thing? I hope so.