From my experience, as mum of teenagers, giving dc smart phones in year 6 means you can control and teach phone use etiquette. You need clear rules and boundaries that are strict and can reduce as you navigate what works for you.
I personally think waiting and giving a 15 yo a smart phone means they won’t listen to a parent’s guidance and you’ve missed the boat in teaching important learning.
My dc are 13 (twins in year 9) and 16 (nearly 17).
rules included:
no social media except WhatsApp
no groups on WhatsApp (that’s where trouble begins ime)
Phone is used for limited time increasing as they get older
when I day “phones down” I mean it and do not expect pushback
phones used downstairs only
put on the family docking station at night
this eased with Snapchat being added at the end of year 8 (I still hate it but dc have proven I can trust them on WhatsApp)
phones are now allowed upstairs for specific reasons and permission has to be requested (eg. Can I phone my boyfriend? Can I listen to music upstairs? type requests).
The advice shouldn’t be “don’t buy dc a mobile” it should, imo, be “make clear rules that are very strict initially and commit to teaching your dc how to use a mobile before purchasing one.”
Giving older teens mobiles and hoping they’ll figure it out is a recipe for disaster imo.