Couples of all ages should practise safe sex, the researchers say
Couples who meet in their 30s or 40s are less likely to use condoms than younger counterparts, a study suggests.
University College London researchers found two thirds of men and women in their late teens used a condom when they first had sex with a new partner.
But only a third of men and women aged 35 to 44 in new relationships did so.
Experts said the International Journal of Epidemiology study showed a worrying trend, and said people should practise safe sex at any age.
The UK's Health Protection Agency has said rates of sexually transmitted infections (STIs) are on the increase.
It reported a rise of 6% in the total number of new STIs diagnosed in 2007 compared with 2006.
Other research from the West Midlands, published this year, found STI diagnoses in people aged 45 more than doubled between 1996 and 2003.
Men have sex 'sooner'
The researchers interviewed more than 11,100 adults as part of the second British National Survey of Sexual Attitudes and Lifestyles.
People were asked a range of questions about their partnerships over the last 12 months including how they met, when they first had sex, what the age gap was and if they used condoms.
Just under 9,600 people reported having 15,488 partners in the preceding year.
A higher proportion of men's partnerships were described as "not regular" - 39% compared with 20% of women's partnerships.
In addition, a higher proportion of women's partnerships were marriages or cohabitations - 55% versus 39% of men's partnerships.
Men had sex sooner after first meeting a partner than women, with one in five men reporting sex within 24 hours of meeting their partner, compared with one in 10 women.
And men who had sex within 24 hours of meeting their partner were more likely to report using a condom than those who waited longer than a day.
Overall, half of all new partnerships involved condom use at first sex, but this declined with age.
In partnerships where there was an age difference of five or more years, condom use is particularly low - especially in relationships between younger women and older men.
And condom use at the end of a relationship was lower than at the beginning.
Just 37% of men had used a condom on the final occasion they had sex with a partner, while the figure was 28% for women.
Dr Catherine Mercer, of the Centre for Sexual Health and HIV Research, University College London, said: "Our finding that condom use at first sex declined with increasing age is of concern.
"Although a disproportionate amount of partnerships are formed among people in their teens and 20s, the fact is that about 45% of marriages are now expected to end in divorce, which means that the 'population attributable risk' of partnership formation by those in their 30s and 40s will increase.
"Indeed, increasing rates of STIs diagnosed among those in their 30s and 40s suggest that interventions that promote consistent condom use with new partners are urgently required, not just for young people as has been the focus recently, but for people in their 30s and 40s and older who are increasingly forming new partnerships."
Julie Bentley, chief executive of the Family Planning Association, said: "This research shows that no matter what age you are, or what type of relationship you are in, sexual health messages still apply."
Lisa Power, of the Terrence Higgins Trust, said: "STIs are no respecter of age and although most media portrayals of sexual relationships involve young people, these figures show that it's important to support people to have a healthy sex life at every age."