While about one-third of Japanese primary school students aged 7-12 years old use cellphones, by the time they get to high school that figure has shot up to 96 percent, according to a government survey released last month.
They are using their phones to read books, listen to music, chat with friends and surf the Internet -- an average of 124 minutes a day for high school girls and 92 minutes for boys.
While the wired world they now inhabit holds enormous advantages for learning and communicating, it also brings a downside, say experts who point to a rise in cyberbullying and a growing inability among teenagers to deal with other people face to face.
"Kids say what's most important to them, next to their own lives, is their cellphone," said Masashi Yasukawa, head of the private National Web Counselling Council.
"They are moving their thumbs while eating or watching television," he said.
The passion in 20-year-old Ayumi Chiba's voice backs up this assertion.
"My life is impossible without it," she says of her cellphone. "I used to pretend I was sick and leave school early when I forgot to take it with me."
Hideki Nakagawa, a sociology professor atin , said cellphones have become "an obsession" for youngsters.
"They feel insecure without cellphones, just the way sales people do without their name cards," he said.
As the multi-faceted cellphone takes centre stage in teen life, it plays a number of roles -- including a weapon that children can wield against each other with no thought for the consequences.
Yasukawa recalls the case of a 15-year-old girl who regularly received messages telling her: "Die," "You're a nuisance" and "You smell".
They turned out to have been sent by a friend in whom she had confided and who told her not to take the messages too seriously.
"The girl who was doing the bullying confessed it made her feel good to see the unease spreading on her friend's face," Yasukawa said.
"Some children send nasty messages to a 'friend' while in her company, pretending to be looking at her profile page on the cellphone.
"It's a very scary world," he said. "Parents don't know there's a very scary world behind cellphone screens."
As they reveal personal information about themselves, children can become prey for fraudsters and paedophiles, as only about one percent have blocks on potentially harmful material.
But on protected sites such as school bulletin boards that do block adults, bullies are free to anonymously post comments without any teacher oversight or intervention.
"Bully-to-bullied relations can be easily reversed with a targeted kid pointing the finger at somebody else for some trivial thing," Yasukawa said, adding that this potentially created "a survival game among children".
's largest mobile carrier . in December launched a line of cellphones for small children, with software ranging from picture books to school scheduling pads aimed at helping them to learn.
The cellphones will eventually become their main means of communication.
Education professor Tetsuro Saito said a survey of 1,600 middle school students aged around 14 found about 60 percent carried cellphones and nearly half used them to send 20 or more emails a day.
Most middle school cellphone users rarely used their phone to talk, the survey found. Saito, of Kawamura Gakuen Women's University near, said children seemed to want the security of communicating with someone, without the bother of dealing with a real person.
"Communication ability is bound to decline as cellphones and other devices are now getting between people," he said.
Tomomi, 18, who would not give her full name, said: "I send some 20 emails a day. There are people I don't talk with -- even if I see them at school, I just exchange mail with them. I guess we're connected only by a machine."
Saito's survey found that students can also use their cellphones as an emotional crutch, and the more problems they have at home, the more dependent they seem to become on their phones.
More than 60 percent of students who said they do not enjoy being with their families send 20 or more emails a day, compared with 35 percent of those happy with their families.
And even if cellphones can bring solace, it can come at a terrible cost.
Kanae Yokoyama, 36, is facing trial for beating and spraining the neck of her 15-year-old daughter after catching her secretly using her cellphone in November.
The girl had been prohibited from using her phone as the bill had hit 120,000 yen (1,060 dollars) in October, mostly wracked up by downloading music and playing games, according to local police.
They said the mother had a history of abusing her daughter."Considering she was often absent from school, the mobile phone may have been her sole 'friend' to spend her days with," a police official said.