Researchers speculated that despite the rise of dating apps such as Tinder young people were having less sex because they were spending more time online, rather than interacting directly with others.
The widespread availability of pornography, high numbers of people living with their parents and concern about being sexually abused were also put forward as possible reasons for the trend.
The study found that 15 per cent of 20 to 24-year-olds born in the 1990s, part of the Millennial generation, reported they had not had a sexual partner since they were 18. The same figure for their predecessors, Generation X, was just six per cent.
The last generation of young people with a higher rate of sexual abstinence was in the 1920s.
One of the researchers, Professor Jean Twenge, a psychologist at San Diego State University, said: “Online dating apps should, in theory, help Millennials find sexual partners more easily.
“However, technology may have the opposite effect if young people are spending so much time online that they interact less in person, and thus don’t have sex.”
She added that media reports about sexual abuse at universities might be putting some people off the idea.
“This generation is very interested in safety, which also appears in their reduced use of alcohol and their interest in ‘safe spaces’ on campus,” Professor Twenge said.
“This is a very risk-averse generation, and that attitude may be influencing their sexual choices.”
She added that young people might also be finding instant entertainment online – not just porn – that was distracting them from wanting to have sex.
The researchers, who reported their findings in a paper in the journal Archives of Sexual Behaviour, analysed data from 26,707 respondents to the General Social Survey, which includes Millennials and members of Generation X.
The pattern appears to be continuing among teenagers in the US. Recent figures from the US Centres for Disease Control’s Youth Risk Behaviour Survey, found the percentage of high-school students who had had sex was 41 per cent in 2015, compared to 51 per cent in 1991.
“This generation appears to be waiting longer to have sex, with an increasing minority apparently waiting until their early 20s or later,” said Professor Twenge.
“It’s good news for sexual and emotional health if teens are waiting until they are ready.
“But if young adults forgo sex completely, they may be missing out on some of the advantages of an adult romantic relationship.”
However, despite the trend Millennials do not appear to disapprove of premarital sex on moral grounds, the researchers said.
Professor Ryne Sherman, a psychologist at Florida Atlantic University and a co-author of the paper, said: “While attitudes about premarital sex have become more permissive over time, rise in individualism allows young American adults to have permissive attitudes without feeling the pressure to conform in their own behaviour.”
By Ian Johnston