About 40 percent of those aged between 17 and 19 think sex education at school is useless, an online survey by the Nippon Foundation think tank showed Thursday.

Almost one in four respondents said they have had sex, and three out of four said they were worried about sexually transmitted diseases. Nearly 90 percent feel contraception is necessary, according to the survey.

In the survey conducted in October targeting 800 young people nationwide, 59.1 percent said sex education at school was useful but the remainder said it was of no help.

One female respondent said schools should explain more about the importance of contraception, while a male said the dangers of sexually transmitted diseases should be given more emphasis.

The questionnaire indicated students were dissatisfied with the lack of specifics taught, with one female student saying many teachers were vague in their expressions when teaching about sex.

For a multiple-choice question asking respondents where they got information about sex, 55.8 percent cited websites, followed by friends at 50.2 percent.

Of the total, 66.7 percent had masturbated, with 74.8 percent of them using smartphones to access explicit content and 8.8 percent simply fantasizing.

Since September, the Nippon Foundation has been surveying teenagers to seek opinions on topics related to adult responsibilities.

The legal age of adulthood in Japan will be lowered from 20 to 18 in April 2022 following a Civil Code revision. The minimum voting age was reduced to 18 after a revised election law came into effect in June 2016.