Contrary to popular opinion, children's brains can cope with phones and tablets from a very young age yet too much screen time may be causing the type of cortex thinning that is usually associated with a more mature age. 

The National Institutes of Health that found out about this phenomenon in kids who are behind touchscreens for more than 7 hours a day, however, is itself not sure that this is a bad thing. For now, the American Academy of Pediatrics' newest guidelines are as follows:

For children younger than 18 months, avoid use of screen media ...