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

Almost one in four respondents said they have experience with sex and three out of four people surveyed said they are worried about sexually transmitted diseases. Nearly 90 percent feel that contraception is necessary, according to the Nippon Foundation.

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

A 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 lessons.

In a multiple-choice question asking respondents where they sourced information about sex, 55.8 percent cited websites, followed by friends at 50.2 percent and social networking sites at 31.4 percent.

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

The Nippon Foundation has been seeking out public opinion via surveys since September on such topics as people being given adult responsibilities, like the ability to sign a contract, at age 18, as well as on love and marriage.

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