STI rates have been increasing among people over 45
Many middle-aged people are continuing to take an irresponsible attitude to their sexual health, say experts.
The Royal Pharmaceutical Society of Great Britain polled over 2,000 adults.
Nearly a fifth of those polled aged 45 to 54 said they had had unprotected sex with someone other than a long-term partner in the past five years.
There is a misconception that their risk of catching a sexually transmitted infection (STI) is "next to nothing", says the RPSGB.
Sexually transmitted infections have doubled in under a decade in people over 45 and have been rising at a faster rate than in the young, recent figures from the Health Protection Agency show.
Older people are increasingly likely to be single or undergoing relationship changes and are less likely to consistently use condoms, perhaps because the risk of pregnancy no longer exists, experts have observed.
The RPSGB's survey of 2,258 UK adults - half who were aged 45 plus - found older generations were flippant about the risks of catching an STI.
A quarter of the 45-54-year-olds surveyed said they did not use contraception as they trusted the person they were sleeping with not to have an STI, with one in 10 saying they did not like the feeling of condoms.
Nearly a third surveyed described their risk of getting an STI when having unprotected sex with a new partner or someone other than their current partner as unlikely or very unlikely.
A further 20% believed that their chances of picking up an infection were "next to nothing".
Double the number of over 55s (25%) believed their chances of acquiring an STI from unprotected sex were next to nothing, compared to 13% of those aged 18-24.
RPSGB spokeswoman Heidi Wright said: "The majority of safe sex messages are targeted at teenagers, but as more adults begin new relationships later in life, they quite clearly need advice too.
"You can't always tell who has an STI and infections don't discriminate on the basis of age."
She said local pharmacies were an excellent source of sexual health advice and support.
Lisa Power of the Terrence Higgins Trust said: "Teenagers aren't the only people who think they're immune from harm.
"Whatever your age, if you have a new sexual partner or more than one, condoms should be a basic part of ensuring your sexual health and theirs.
"It's more embarrassing to get an infection than to use a condom."
A Department of Health spokesperson said: "The message is the same for everyone - anyone having unprotected sex potentially puts themselves at risk of an STI.
"Infections rates have risen in all age groups, including older people.
"Older people also need to be aware of the need to use condoms consistently, particularly those who are newly single and entering relationships with new partners."