Completely ridiculous and impractical to ban the internet from those under 18. Imagine 17 year olds not able to use a phone to look up bus or train information, read the news, listen to music, research for their homework, look for university courses and research a city, use a map application, look for jobs, do online banking, order a book, buy themselves clothes, book the cinema, buy train ticket etc. All things my DD has used the internet for. My 17 year old spent 5 weeks interailing round Europe by herself, but you think she's too young to have a phone? She wouldn't have been able to even plan her trip without access to the internet and without the practice of using it in the preceding years- knowing how to research, how to pay for things online, book trains, check her bank balance, use a navigation tool etc.
You can't separate the adult and child world, it has to be a gradual introduction, starting with supervised access and educating them about how to keep themselves safe, how to conduct themselves online, when to ask for help and report things.
Imagine the difficulties 18 year olds would have if they were denied devices and then given them at 18 and the problems it would cause. They wouldn't be able to do jobs involving the internet or social media as they would be ignorant to i. And imagine being a 17 year old with an August birthday in year group full of friends 18 year old friends with social media accounts which you can't access.
Stupid idea!