I disagree:
Firstly, most issues you mention (cyberbullying, radicalisation... etc) won't be stopped just by banning phones. It will just move onto computers. Banning mobile phones isn't a solution to those issues. And then what? Ban computers? In countries that aren't run by repressive dictatorships or 1984-esque; that simply won't go down well. People in the UK are used to certain luxuries like internet, tv, mobiles etc, if someone suddenly initiates a national ban on under 18 possession there'll be a riot somewhere. Even if you ban computer use for under 18's, how do you realistically enforce that? Unless you run a tight surveillance state on the same level as North Korea, then there are so many ways under 18's could access computers. Simply put, banning technology will not solve the issues you present.
Secondly, mobile phones are incredibly handy. The need to contact people so easily is ridiculously useful. Unlike phone boxes, mobile phones can be on your person. No matter where you are, you have an instant way of contacting some one. In a remote location and need emergency assistance? Just dial 999, you don't need to go looking for people or a phone box to get help.
Thirdly, just because we use a phone doesn't mean we are socially screwed. With phones, we can socialise to people from any point on the globe. In the train? Well, you can still chat to friends and family and etc... But with everything, mobile phones comes with advantages and disadvantages, one disadvantage is that face to face socialisation may decrease. Plus, you shouldn't generalise a whole generation. For example, there are stories of people on dates or social gatherings where they pay more attentions to their phones than the people they're with, but that isn't everyone. People in that same age demographic call it out and talk about how much they loathe that kind of behaviour, there are some that simply do not participate in that behaviour.
In general, we the subject of cyberbullying is bought up, the first thought is literally how it happens at home on a facebook on a computer. In schools, whenever teachers talk about it almost always is about how it happens on social media. Social media is not restricted to a phone. Yes if you ban mobiles for under 18's, cyberbullying via snapchat and apps that don't really work on computers may stop... but so will those social media apps. The snapchat users will move to facebook or something and the cyberullying continues.