Here's what you should do: Check how much a 6s is. If they want an iPhone 8, make them safe their money, and add the price of the 6s to their savings. Believe me, the phone will last a lot longer if they had to save their own money for it.
Does the teen have a job and pay their portion of the bill?
If not they should be thankful. Maybe I’m old fashioned. I’m 38 and didn’t have a phone until I had a job and paid my own bills at 18.
I know when my kid gets one it will be the iPhone SE I have put aside and that will be going into middle school. Which is what our group of parents all agreed on so there is no “but she got a phone” fights
This is why my son only got an 8GB iPhone 4s when he was 13 with a 2GB data line.
We gave him a phone at that age because I go one way to work, my wife goes another and at the time my two kids went to the same school. We needed to be able to get a hold of him.
He was told that if he did okay, in a year we'd upgrade him. He did very well and so in 6 months he got my wife's iPhone 5 and I increased his line to 6GB.
By his 14th birthday there had been zero issues. He's an 'A' student in a STEAM school and has never lost his phone to the teacher. So, we got him an iPhone SE brand new.
He is still doing well. I have the old 4s now, my daughter has my wife's old 5 (no cellular and it stays at home because she's 10). A year later and there has been zero issues with him and the phone.
My point is that each child is different and you have to judge. There is no way in hell I am giving my daughter a cellular enabled phone right now. Probably not even until she's 16 or so. She isn't responsible yet.
However, I would just add that my son has had a laptop since he was 5 (and my daughter since she was 7 or 8). That's meant that they didn't really miss a phone. That said, lest anyone think the laptops are where I made up for old phones…well the first one was an iBook G3 with a bad airport antenna. Next was a PowerBook G4 TiBook and an Aluminum PowerBook G4 with a bad ram slot. My son's current laptop is a 9 year old Thinkpad with Windows 7.