If the same increase was seen in whole organisms as was seen in the cells, then, Dr Lanza said: "For a human who might naturally be able to live for 120 years, they could very well live to 200."
The fact that the process of cloning appears to reverse ageing is a major surprise as Dolly, the cloned sheep, appears biologically older than her age.
They hope that in the future cloning will enable a patient's own tissue to be used to grow compatible transplant tissue which would also be youthful. Such tissue could be used to treat diseases of ageing such as Alzheimer's, arthritis and heart disease.
Turning back the clock
The cloning process used by the US and Canadian team seems to have literally turned back the ageing clock in the cells of the six heifers.
One of the researchers, Robert Lanza of Advanced Cell Technology, Massachusetts, told the BBC: "There is a very real possibility that, if this cellular phenomenon transfers into an entire organism, you would end up with a patient or an animal that had a longer life span."
If the same increase was seen in whole organisms as was seen in the cells, then, Dr Lanza said: "For a human who might naturally be able to live for 120 years, they could very well live to 200."
However, the research breakthrough will lead to renewed questions over the ethics and safety of cloning animals or humans, as well as the ethics of extending the lifespan of people.
Dr Lanza said: "We think it would be premature and not safe to apply our work to humans at this point and not ethical to use it for any other purpose than therapeutic reasons, i.e. to alleviate suffering from disease."
Worn out
Scientists calculate the biological age of cells by looking at telomeres. These are little caps on the ends of the chromosomes that carry the genetic blueprint inside cells.
Each time a cell divides, the telomeres become a little worn. When they are frayed beyond repair, the cell dies.
When Dolly was born in 1997, her cells appeared to be the same age as the cells of the 6-year-old ewe from which she was cloned.
However, the six heifers, cloned by taking cells from a 45-day-old foetus and placing them in an egg, have exceptionally young telomeres.
Time machine
"The egg cells acts like a little time machine and can take it back, as far as we can tell, to the beginning of life," said Dr Michael West, president of ACT.
He believes the medical implications are far-reaching: "It's the first day in a new era in treating age-related disease. We could take one cell from a patient, make hundreds or thousands of young cells and give them back a young immune system or give them back young cartilage in their knees."
However, producing those cells will almost certainly involve creating cloned human embryos. These would not be allowed to grow into babies but there remains strong opposition from some groups.
The UK anti-abortion charity Life said it was worried about some of the implications of the research.
National chairman Jack Scarisbrick said: "If this research hastens the drive towards cloning using embryonic cells, which inevitably causes the death of a human being, then we would deplore it.
"We are totally opposed to cloning of human beings for any purpose whatsoever."
Donor cells
Research is continuing to pin down why cloning bequeathed the US cows younger cells, whereas Dolly was left with older cells.
One factor could be the stage reached by the cell used for cloning. The cells used to clone the cows were still dividing, though near the end of their life cycle. Dolly's creators at the Roslin Institute in Edinburgh, UK, used cells that had been starved and sent into a resting state.
Dolly was also produced from a mammary cell whereas connective tissue cells called fibroblasts were used to produce the cows.
Dr Lanza said: "Previous studies have indicated that there may be variation in how different cell types repair telomeres, which could make the choice of donor cell significant."
The research is published in the journal Science.