Nearly one in 10 schoolchildren gets their first mobile phone by the time they are five, a new study suggests.
The average child is given a handset at the age of 11 but nearly one in 10 has a phone at less than half this age.
Parents spend an average £125 on their youngster's gadgets - and around £246 on their own mobiles, the study found.
But despite the cost, more than four in 10 adults (42%) do not pay close attention to their children's phone bills and just a quarter (25%) cap their contracts.
Children spend an average £11 per month on their mobiles - less than parents who spend £19, the survey revealed. But more than one in 10 (11%) youngsters spends more than their mother or father.
The study, by comparison site uSwitch.com, found children were, on average, given their first mobiles soon after starting secondary school at 11 years and eight months. Some 9% of parents said they bought their youngsters a phone when they were five-years-old.
Ernest Doku, telecoms expert at uSwitch.com, said: "As well as arming kids with mobiles for emergencies and peace of mind, I'd imagine that many parents have bought their kids smartphones just to stop them commandeering their own when bored."
He suggested parents place caps on their children's mobile bills. "Make sure that when they're at home, your kids are browsing the web using wi-fi instead of consuming data by connecting to the internet via 3G or 4G," he added.
According to the study, parents are likely to spend more money on their first-born's phone and bills than on those of any younger siblings.
Just a quarter of parents cap their children's contracts but fewer than one in 20 (3%) disable the data function on their phones so they are only able to use them to call and text.
Some 1,420 parents with children aged under 16 were consulted for the study.