This is a very broad and at the same time an important. A mobile phone can have multiple effects, specially a smartphone. A phone connected with internet can help gaining ng lot of information and personality development for a growing up kid. Learning new language, studying, staying update, showing up creativity, socializing on social media… Possibilities are endless but on the other hand there are multiple effects of using a smartphone including physical and mental.
Effects of radiation are still under study and blue light is already known for interfering with sleep patterns. Too much dependency on internet leads to less efficient brain e.g. poor memory. Kids might be also exposed to dangers of pornography, cyber bullying etc.
I feel that kids must be mature enough (min. 13-16) unless you hand them over a phone and parents must still keep a eye on them. Below that age, kids must not own a smartphone and any usage must be under adult supervision only
