The Journal of the Formosan Medical Association graciously had released the following under Open Access.

The overall prevalence of MP use for Taiwanese children aged 11–15 years was estimated at 63.2% (95% CI = 61.1–65.3%). The prevalence showed a small sex difference, but presented evident age and geographic variations. The prevalence increased steadily from 45% for 11-year-old children to 71.7% for children aged 15 years. Children living in the Central area showed the highest prevalence (69.3%), whereas those from the South area had the lowest prevalence at 58.3%. We also noted that children who attended private schools had a higher prevalence of MP use than public school students.

Some 71.1% of guardians reported that the main reason for their children to use MPs was because of safety considerations. However, 27.6% admitted that peer pressure was the main reason for their children to own MPs. Forty-five percent of children used MPs for calling or receiving every day, 30.7% talked >2 days/week, and 18.9% used MPs 1–2 days/week. More than half (45%) of children had used MPs for calling or receiving daily, 34.8% reported daily MP use of 21–40 minutes, and 4.4% of children used MPs for at least 1 hour every day. During weekdays, children often (41.7%) talked on MPs in the evening. The MP use pattern during the weekend was somewhat different; children often used their MPs for calling or receiving in the afternoon (33.3%), and then in the evening (24.6%). Of the children studied, 9.8% frequently used MPs for calling or receiving after 10:00 PM, a time of lights out for many families during weekdays, and the corresponding figure for the weekend was 6.7%
From Mobile phone use and health symptoms in children in the Journal of the Formosan Medical Association
Volume 114, Issue 7, July 2015, Pages 598-604