WASHINGTON — AIDS is graying. By the end of the decade, the government estimates, more than half of Americans living with HIV will be over 50. Even in developing countries, more people with the AIDS virus are surviving to middle age and beyond.

That’s good news — but it’s also a challenge. There’s growing evidence that people who have spent decades battling the virus might be aging prematurely. At the International AIDS Conference this week, numerous studies are examining how heart disease, thinning bones and a list of other health problems typically seen in the senior years seem to hit many people with HIV when they’re only in their 50s.

“I’m 54, but I feel older,” said Carolyn Massey of Laurel, Md., who has lived with HIV for nearly 20 years. “When I hear young people talk about, ‘Well you get HIV and you take your drugs and you’ll be all right’ — that’s just not the truth. This is a lifelong thing we’re talking about, and it unfolds every day on you.”

The graying isn’t just because people like Massey are living longer. Some of it comes from older adults being newly diagnosed, a trend U.S. health officials say is small but slowly growing.

“They let down their guard,” is how Dr. Kevin Fenton of the Centers for Disease Control and Prevention puts it.

Already, a third of the nearly 1.2 million people living with HIV in the U.S. are over 50. By 2020, half will be, Fenton said at one of numerous sessions on aging at the world’s largest AIDS meeting.

People 50 or older accounted for 17 percent of new HIV diagnoses in 2009, according to the CDC’s latest data. That’s up from 13 percent in 2001.

There aren’t as good counts in poor regions of the world, where access to medications came years later than in developed countries.

But even in hard-hit sub-Saharan Africa, home to most of world’s HIV-infected population, studies suggest 3 million people living with HIV are 50-plus, said Dr. Joel Negin of the University of Sydney in Australia. By 2040, he said, that could reach 9 million.

One-third

People in the U.S. living with HIV who are 50 or older

17%

Percentage of new HIV diagnoses in the U.S. in 2009 that are people 50 or older

3 million

HIV-infected people in sub-Saharan Africa who are over 50