One in five new cases of HIV in Northern Ireland are in men over the age of 45.
The figures have been rising for the past two years.
They are causing concern among doctors who are trying to reinforce the safe sex message.
Doctors at the Royal Victoria Hospital are also running clinics in gay clubs to try to convince hard to reach groups of the importance of testing.
Dr Carol Emerson, a consultant specialising in sexual health at the Royal Victoria Hospital, said the outreach clinics in gay venues in Belfast were essential.
"I'm really passionate about reaching people and ensuring they get HIV testing and full sexual health advice," she said.
"Some people find it a step too far to come to a GUM (sexual health) clinic and some people really regard confidentiality as the holy grail and bringing the clinic out has increased testing, increased knowledge and increased diagnoses."
Dr Emerson said there were many reasons why people did not want to come forward - not least the continued stigma about being gay in Northern Ireland.
Some patients are also married and have sex with men outside that relationship.
Others have simply ignored the safe sex messages and are too frightened to come forward.
"We are seeing a range of people over 45 testing for different reasons and it may be that they've had a stable relationship for a period of time and that has broken down and now they've found a new lease of life and they've picked up a new infection and that's not just HIV - it's the whole spectrum of sexual health," she said.
The clinics run once a month at various different venues frequented by members of the gay community.
And there is an additional worrying trend.
Around half of those who are diagnosed need to go onto treatment right away because the condition has progressed to a serious extent.
"Their immune system isn't as strong as it might have been and they are being strongly advised to start the HIV treatment right away," Dr Emerson said.
The Rainbow Project, which lobbies for gay, lesbian and transgender rights, also carries out free testing.
Its director, John O'Doherty, is also worried about the increase in HIV in the over-45 age group.
"We're dealing with a community which spent a large part of their adult years being a criminal based on their sexual orientation," he said.
"So the new freedom and openness and visibility of our community has provided a lot more opportunities for relationships and to meet new people, so it does put them at increased risk."
The Kremlin club is Belfast's oldest and best known gay venue. It is one of a number of premises to host the clinics and Mr O'Doherty and his team often leaflet clubbers and hand out safe sex packs.
Philip Alexander and Matthew Armstrong are from Ballymena.
They are in their early 20s and often travel to Belfast for a night out in the Kremlin.
Matthew said his generation would be different.
"Good sexual health is something everyone should consider. You should put your health before anything," he said.
Philip added: "I think everyone of our age is worried about image and health.
"We are more health conscious. Definitely there are people out there who don't take it seriously - think there's no harm - but most of us are aware of the safe sex message."