The number of underage sex offenders in Taiwan has increased four-fold over the past five years, with the youngest aged only seven, a study showed Thursday.

In 2008, 751 of of 6,227 sex offenders in total were under 18, compared with 153 in 2005, according to the study from the Taiwan Fund for Children and Families.

The youngest alleged offender was a seven-year-old boy accused of sexually harassing a three-year-old girl next door after watching his father's adult movie, said the foundation, a non-profit organisation devoted to child welfare.

In the first nine months of this year alone, the number of child offenders reached 730, according to the fund.

The study, citing official data, also found an alarming trend as the number of offenders below the age of 12 as well as female minor offenders had both doubled in five years.

As with adult violators, most of the troubled youngsters come from broken or dysfunctional families, driving them to become more violent, aggressive and impulsive, it said.