NOT long ago, many parents wondered at what age they should give their child full access to the car keys. Nowadays, parents face a trickier question: At what age should a child own a smartphone? The smartphone, after all, is the key to unfettered access to the internet and the many benefits and dangers that come with it. But unlike driving a car, which is legal in some states starting at the age of 16, there is no legal guideline for a parent to determine when a child may be ready for a smartphone.