While there are still many unanswered questions on the long-term effects of smartphones and social media on children and teenagers, existing research provides some evidence on their main risks and benefits.
According to recent research, collated and published by the BBC, there is no overarching evidence showing that owning a phone or using social media is harmful to children's wellbeing in general. But the research has so far only focused on teenagers, while there is also emerging evidence that shows there may be specific developmental phases where children are more at risk from negative effects.
Data from Ofcom, the UK's communications regulator, show that the vast majority of children in the UK own a smartphone by the age of 11, with ownership rising from 44% at age nine to 91% at age 11.
In the US, 37% of parents of nine- to 11-year olds say their child has their own smartphone. And in a European study across 19 countries, 80% percent of children nine aged to 16 reported using a smartphone to go online daily or almost daily.
"By the time we get to older teens, over 90% of kids have a phone," Candice Odgers, professor of psychological science at the University of California, Irvine, in the US, told the BBC.
While a European report into digital technology use among children from birth to eight years old found that this age group had "limited or no perception of online risks", when it comes to the detrimental effects of smartphone use – and social media apps accessed through them – on older children, solid evidence is lacking.
Odgers analysed six meta-analyses looking at the link between digital technology use and child adolescent mental health, as well as other large-scale studies and daily diary studies. She found no consistent link between adolescents' technology use and their wellbeing.
"The majority of studies find no association between social media use and mental health," says Odgers. In the studies that did find an association, the effect sizes – both positive and negative – were small. "The biggest finding really was a disconnect between what people believe, including adolescents themselves, and what the evidence actually says," she says.
Another review, by Amy Orben, an experimental psychologist at the University of Cambridge, UK, also found evidence inconclusive.
While according to the report published in the BBC, there was a small negative correlation, on average, across the studies included, Orben concluded it was impossible to know whether the technology was causing the dip in wellbeing or vice versa – or whether other factors were influencing both. Much of the research in this area is not of high enough quality to deliver meaningful results, she notes.
Of course, these results are averages. "There's an inherent large variation around that impact [on wellbeing] that has been found in the scientific literature," says Orben, and the experience of individual teenagers will depend on their own personal circumstances. "The only person who really can judge that is often the people who are closest to them," she adds.
In practical terms, this means that regardless of what the broader evidence says, there may be children who do struggle as the result of using social media or certain apps – and it's important for parents to be attuned to this, and offer support.
But it can also be a useful tool for children who are in doubt, yet might be ashamed to publicly disclose their concerns.
"Imagine you are a teenager worried that puberty is going wrong, or your sexuality isn't the same as your friends', or worried about climate change when the adults around you are bored with it," says Sonia Livingstone, professor of social psychology at the London School of Economics, UK, and co-author of the book Parenting for a Digital Future told the BBC.
For the most part, though, when they're using their phone for communication, children are talking to friends and family. "If you actually analyse who kids are talking to online […] there's a very strong overlap with their offline network," says Odgers. "I think this whole idea that we're losing a kid in isolation to the phone – for some kids, that can be a real risk, but for the vast majority of kids, they're connecting, they're sharing, they're co-viewing."
Analysing data from over 17,000 participants aged between 10 and 21, the researchers found that higher use of social media at ages 11 to 13 for girls, and 14 to 15 for boys, predicted lower life satisfaction a year later. The reverse was also true: lower social media use at this age predicted higher life satisfaction the following year.
As well as age, other factors could influence the impact of social media on children and teenagers – but researchers are only just beginning to explore these individual differences.
The key question parents need to ask, says Odgers, is: "How does it fit for the child and for the family?"
For many parents, buying a child a phone is a practical decision. "In a lot of cases, parents are the ones that want the younger children to have phones so that they can keep in touch throughout the day, they can coordinate pickups," says Odgers.
It can also be seen as a milestone on the road to adulthood. "I think for children it gives them a sense of independence and responsibility," says Anja Stevic, researcher in the department of communication at the University of Vienna, Austria. "This is definitely something that parents should consider: are their children at a stage where they are responsible enough to have their own device?"
Even very young children learn from their parent's phone use. A European report into digital technology use among children from birth to eight years old, found that this age group had little or no awareness of the risks, but that children often mirrored their parents' technology use. Some parents even discovered during the study that children knew their device passwords, and so could access them independently.
But parents can use this to their advantage by getting younger children involved during smartphone-based tasks, and modelling good practice. "I think this involvement and co-use, that's actually a good way for them to learn what's happening on this device, what it is for," says Stevic.
Ultimately, when to buy a smartphone for a child comes down to a value decision for parents. For some, the right decision will be not to buy one – and, with a little bit of creativity, children without a smartphone don't have to miss out.