People in their 30s and 40s are much less likely than teens to use condoms when having sex with someone for the first time, new research from the UK shows.

While 68 percent of 16- to 19-year-olds said they used protection for the first sexual encounter with a new partner, just 38 percent of men and 29 percent of women aged 35 to 44 reported doing so, Dr. Catherine H. Mercer of University College London and her colleagues found.

"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," Mercer and her team write in the International Journal of Epidemiology.

The researchers surveyed 11,161 men and women 16 to 44 years old. Each person was interviewed face-to-face, and then answered more sensitive questions about sexual issues using a computer-assisted self-interview. Study participants reported the number of sexual partners they'd had, and also gave detailed information on the nature of their last three relationships.

The survey turned up 15,488 heterosexual partnerships reported by 9,598 people. For men, 39 percent of these partnerships were casual (defined as "not (yet) regular"), while 20 percent of women's were. Overall, 37 percent of men and 29 percent of women said they used a condom the last time they had sex.

When having sex for the first time with a partner, 55 percent of participants used a condom. Couples with an age difference of five years or greater were less likely to use condoms when they had sex the first time. And 23 percent of men said that they'd had sex within 24 hours of meeting their current partner, compared to about 11 percent of women.

The high rate of unprotected sex they identified, especially in casual partnerships is "worrisome," the researchers say. They conclude: "Condom use with new partners needs to be promoted among all age groups."