DS had his 1st mobile phone when he started at secondary school. Used it from the moment he got it, would text me to tell me he was running late or had missed the bus yada yada, upgraded his phone a few times now and is never without it. I can always contact him should I need to.
Nephew given a phone when he started secondary too, doesn't use it as much but can always contact him if you need to and although he doesn't tend to rely on it as much as DS relys on his, he would still be stuck without it.
Niece given a phone when she was 10, has no one other than her parents to text/call on it, so it stayed in her room at home most of the time and she played games on it. She hasn't actually charged it up since October and it sits gathering dust in her room.
Younger niece was given a phone when she was 7, and it has been charged once. It has had £5 credit on it for the last 4 months and niece has never texted or phoned anyone. It was charged up once and niece played the games on it, then the charge ran low, it ran out of power and niece has never picked it up since.
If you buy a phone for your child to use as a phone, to be able to contact people if they need to, to allow people to contact them, then 7 is too young imo.
I remember my DS asking for a phone when he was about 8. I asked him who he would be texting?? He replied 'My best friend.' I reminded him his best friend didn't have a phone, so who else could he text/phone??
He realised the only person he could phone or text would be me, but he was always with me unless he was at school. No point imo.