If sex is too soon, it is the greatest regret for young people who lose their innocence.
More than a third of women and a quarter of male teens and early 20th century admitted that it was not "the right time" when they were first born.
People must have 16 or more to legitimately agree to sex.
The latest national survey on sexual attitudes and lifestyle surveys says that many people at this age may not be ready.
The ten-year Natsala survey provides a detailed picture of sexual behavior in the UK.
This latest work, published by BMJ Sexual & Reproductive Health, London Hygiene and Tropical Medicine School researchers, looked at nearly 3,000 young people who completed the survey between 2010 and 2012.
The answers showed that almost 40% of young women and 26% of young people could not have had their first sexual experience "at the right time".
When asked more deeply, most said they wanted them to have waited longer to lose their innocence. Some said they would have done it faster.
Most were born until they were 18 years old, who did it by the end of 16 years.
Almost a third was sex before turning 16.
The survey also looked at sexual competence or readiness – whether a person could reasonably make a decision about sex for the first time. For example, they had to be sensible enough to have agreed and should not act with pressure.
About half of the young women and four of the ten young men who responded failed this event.
And almost one in five women and one in ten men said they and their partners were not equally prepared for sex at the time, suggesting that some feel compelled to have sex.
Prof. Kaye Wellings, founder of the Nazi survey, said that the age of consent is not an indicator that someone might be ready for sexual activity. "Every young person is different – some 15-year-olds may be ready, but some 18-year-olds are not."
Together, researcher Dr. Melissa Palmer said: "Our results show that young women are more likely than young men to be subjected to pressure from their partners.
"Although the results of the survey provided some positive results, for example, nearly nine out of 10 young people who used safe contraception in the first sex, additional efforts are needed to ensure that young people's welfare is protected because they become sexually active."
She said that sexual education in schools should provide young people with the right negotiating skills to have a safe and positive first sexual experience.
When is the right time?
If you think you might have sex, ask yourself:
- Does it feel right?
- Do I like my partner?
- Does he love me just as much?
- Have we talked about using condoms to prevent STI and HIV, and was it good?
- Have we received contraception to protect against pregnancy?
- Do I think I can say 'no' at any time if I change my mind and are both good?
If you answer yes to all these questions, the time may be right. But if you answer yes to any of these questions, you may not have:
- Do I feel under pressure from anyone like my partner or friend?
- Can I then regret it?
- Do I only think about sex to impress my friends or follow them?
- Do I think about sex only to save my partner?
Source: NHS Choice
Isabel Inman of the Sexual Health Charity Brook said: "We are convinced that age-appropriate relationship and sexual education (RSE) should start early to give young people the opportunity to make positive decisions that are appropriate for them. "