Scientists believe the drug, metformin, may mimic the effects of extreme calorie restriction.
This regime, which is based on eating a very low calorie diet, is thought to promote healthy ageing.
The human implications of the study are unclear, the researchers report in the journal, Nature Communications.
Rafael de Cabo, of the National Institute on Ageing in Baltimore, Maryland, US, said calorie restriction in laboratory animals had been shown to increase their lifespan.
His team is searching for interventions - such as a drug - that can mimic these effects.
Metformin is one of the most widely prescribed treatments for type-2 diabetes, which occurs mainly in people above the age of 40. It is also used to treat metabolic syndrome, a combination of diabetes, high blood pressure and obesity.
Previous work has shown that metformin can extend the lifespan of simple organisms such as worms, but studies in flies and mammals have given conflicting evidence.
The scientists gave one of two different doses of metformin to middle-aged male mice and found that lower doses increased lifespan by about 5%, and also delayed the onset of age-associated diseases. But they said the higher dose of metformin was toxic and reduced the lifespan of mice.
Further studies were needed to determine if metformin has any effect on human health and lifespan, said Dr de Cabo.
"These are very promising results that need to be translated to humans via clinical studies," he said.
He said the best current advice was to eat a good diet and exercise.
"Right now the best that we can say is probably what your grandmother told you," he told BBC News.
"Eat a good diet and exercise are the only two things that we know for sure that they work very well in humans."
Prof Tom Kirkwood, associate dean for ageing at Newcastle University, said it is unclear what the study might mean for human health.
"Metformin is a well-established drug that acts on metabolism and has long been used against type-2 diabetes," he said.
"We've known for a long time that modulating metabolism in mice can extend survival and postpone age-related conditions, and there are sound reasons why this should be the case is a small, short-lived animal.
"What we don't know however is whether similar effects on lifespan might be produced in humans.
"This is something that we cannot simply take for granted and the study's authors do well to sound a note of caution."

The 32-year old, who has won 12 caps for his country, has been signed to provide injury cover.
Ospreys tight heads Dmitri Arhip and Ma'afu Fia are currently sidelined and Rhodri Jones is in the Wales squad.
"Brian has proven quality at the top level and will add so much experience to the squad, particularly over the coming weeks," coach Steve Tandy said.
Mujati has played in Super Rugby with Lions and Stormers, in Top 14 in France with Racing Metro, and in England with Northampton Saints and Sale.
At Northampton Mujati was a losing Heineken Cup and Premiership finalist, and an Anglo-Welsh Cup winner.
Ospreys Head Coach, Steve Tandy, said he is expecting Mujati to make a big impact.
"This is an important signing for us given the injury situation with Dmitri and Ma'afu. Our performances this season have put us in a really strong position but we still have eight PRO12 games to go, and a Challenge Cup quarter-final in April," he said.
"In Rhodri Jones and Daniel Suter we have two young men who will be heavily involved but we can't go through such a big run of games with only two fit pro tight heads on our books, particularly with Rhodri still a part of the Six Nations squad and potentially away on Wales duty. It's a critical position and we felt that we had to act now."
A diabetes pill has anti-ageing effects and extends the life of male mice, research suggests.
[NEXT_CONCEPT]
Ospreys have signed Sale Sharks' former South Africa tight head prop Brian Mujati until the end of the season.