The number of older people with sexually transmitted infections such as chlamydia and gonorrhoea has doubled in the past 10 years.
Research published in the Student British Medical Journal also revealed that men taking medication for erectile dysfunction, including Viagra, are more likely to contract an STI.
Post-menopausal women are more vulnerable to STIs because of physical changes.
Cases of syphilis, chlamydia and gonorrhoea have increased in Britain, the US and Canada among 45 to 64-year-olds, according to the study.
There has also been an increase in cases of HIV among older people, with those aged 50 and over accounting for 20 percent of adults seeking medical care - an 82 percent increase on 2001 figures.
There is no substantial evidence to indicate why there has been such an increase in the number of STIs among those in the 50 to 90-year-old age bracket.
But experts have speculated that a rise in the number of divorces and greater sexual activity with more partners, partly because of more use of Viagra, are partly to blame. People are also living longer, increasing the chances of picking up a sex disease.
There is also the assumption that safe-sex messages are being ignored by older people because they are assumed to be aimed solely at the young.
Rachel von Simson, a medical student at King's College London, and Ranjababu Kulasegaram, consultant genitourinary physician at St Thomas' Hospital London, who wrote the report, urged GPs not to shy away from discussing safe sex with patients, regardless of their age.
"Without a great deal of evidence about the reason for the increase in sexually transmitted infections, it is difficult to know what strategies will work to raise awareness and control the spread of infection," they said.
"Last year, the Family Planning Association ran the first ever national campaign promoting safe sex among the over-50s. Perhaps we need more of those to raise awareness,."
The study revealed that four fifths of 50 to 90-year-olds remain sexually active.