I'm 19 and have had my phone in my bedroom since I got it at maybe 11 or 12.
I have always used it as an alarm, so it was just easier. I would say I am fairly responsible, so I have never done anything bad. I would say the worst thing would be getting into the habit of watching movies to fall asleep.
I don't think it is anywhere near needed until they have a license for one reason. My friend who is a year older was drunk at a party when I was 17, she was 18. Being her only friend who she knew was sober (her parents wouldn't answer) she called me to come get her because something had happened. I know it seems like a rare thing, but it has happened to both me and my brother, as well as news of our friends being in accidents in the middle of the night. It sounds stupid until your kid has potentially saved another person's life. So just take that into consideration.