The society's said that people were found to have a misconception that their risk of catching a sexually transmitted infection (STI) was "next to nothing".
The experts observed that older people were more likely to be single or undergoing relationship changes, and less likely to consistently use condoms, perhaps because the risk of pregnancy no longer exists.
The RPSGB's survey revealed that older generations were flippant about the risks of catching an STI.
One-fourth of the people surveyed said that they did not use contraception because they trusted the person they were sleeping with not to have an STI.
About 10 per cent said that they did not like the feeling of condoms.
The surveyors further revealed that about a third of the subjects described their risk of getting an STI via unprotected sex with a new partner or someone other than their current partner as unlikely or very unlikely.
A further 20 per cent thought that their chances of picking up an infection were "next to nothing".
About 25 per cent of those over 55 believed that their chances of acquiring an STI from unprotected sex were next to nothing, compared to 13 per cent of those aged 18-24.
"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," the BBC quoted RPSGB spokeswoman Heidi Wright as saying.
"You can't always tell who has an STI and infections don't discriminate on the basis of age," Wright added.
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.
The spokesperson added: "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."