Educational professionals recommend that children shouldn't be given their own smartphones until they are at least between 11 or 13 years of age. This is due to the way that smartphones may impair your child's development; children learn primarily from real-world interactions and experiences, and as such if they are tapping away at a screen, indulging in what many of us would easily categorise as escapism, then they simply aren't going to receive the same bountiful upbringing that they might otherwise receive.
Electronic media can be very beneficial from an educational point of view, especially after the child is over 4 or 5 years old, as it can boost their understanding of their school studies, however having a smartphone 'on tap' before the teenage years is not a good situation.
Even if you are the parent of a teenager, you should exercise some control over what your child accesses via their smartphone. Pornography, adult comedy, violent and extreme content; all of these things are easily and readily accessible from any search engine. If kids are able to access online gaming sites like Betway Bingo, which are advertised on television and via the net, then they might end up playing too much, racking up bills they simply can't pay. With all of these potential issues, parents can simply set up parental guidance locks via a number of free apps that can readily be downloaded. This will allow parents to block sites and content that they don't want their children viewing.
It's also a really good idea to talk to your child about the merits and problems surrounding smartphones. If your child is made aware that a truly screen-centric life isn't the norm and will impair their development, they will understand the need the exercise some constraint in their digital activities. If all else fails, apps are also available that allow you to set limits on how long a child can use the internet for each day.