Research shows young people are engaging in more extensive sex with their opposite sex partners, including anal sex.
Experts looked at the responses to a national survey conducted every 10 years in the UK since 1990.
More than one of the ten millennials said they had tried anal sex at the age of 18.
By the age of 22 to 24, three out of 10 said they had tried it.
However, vaginal and oral sex is still the most common type of sexual activity between young men and women.
The age when young people start sex-
Vagina, anus, or mouth
It has not changed much in recent decades.
This was 16 in the most recent survey.
While research in the Journal of Adolescent Health shows what kind of sex people have, it doesn't explain why preferences change.
Experts can only guess, but they say society is receiving more and less sexual experiments and judging less.
Kaye Wellings, professor and senior author of Sexual and Reproductive Health at the London School of Health and Tropical Medicine, said: "The practical changes we are seeing here are consistent with the expansion of other aspects of the sexual experience of young people, taking into account the rapidly changing social environment and the ever-changing
The impact on sexual behavior is increasing.
"Professor Cynthia Graham is a professor of sexual and reproductive health at the University of Southampton.
She said the Internet and the media may play a role in breaking sexual taboos.
"The internet means that people can easily find and see things they could not find in the past.
"Anal sex is still a shame, but attitude seems to be changing," he said.
We know that the society as a whole is getting more and more receptive to same-sex behavior.
But there are few studies on anal sex and motivation.
She said more research is needed to inform sex education and to provide young people with the information they need for sexual health.