US scientists have performed a dramatic reversal of the ageing process in animal studies.
They used a chemical to rejuvenate muscle in mice and said it was the equivalent of transforming a 60-year-old's muscle to that of a 20-year-old - but muscle strength did not improve.
Their study, in the journal Cell, identified an entirely new mechanism of ageing and then reversed it.
Other researchers said it was an "exciting finding".
Ageing is considered a one-way street, but now researchers at Harvard Medical School have shown that some aspects can be reversed.
Their research focused on a chemical called NAD. Its levels naturally drop in all cells of the body with age.
The team showed this disrupted the function of the cell's in-built powerstations, mitochondria, leading to lower energy production and ageing.
Experiments showed that boosting NAD levels, by giving mice a chemical which they naturally convert into NAD, could reverse the sands of time.
One week of youth-medication in two-year-old mice meant their muscles became akin to those of a six-month-old in terms of mitochondrial function, muscle wastage, inflammation and insulin resistance.
Dr Ana Gomes, from the department of genetics at Harvard Medical School, said: "We believe this is quite an important finding."
She argues muscle strength may return with a longer course of treatment.
However, this could never be a cure-all for ageing. Other aspects such as shortening of telomeres or damage to DNA would not be reversed.
Dr Gomes told the BBC: "Ageing is multi-factorial, it's not just one component we can fix, so it's hard to target the whole thing.
"I believe there is a lot of cross-talk in cells and energy is very important in a cell and likely to be a very big component of ageing that might cause some of the other things that happen with ageing."
The research group wants to begin clinical trials in 2015.
Dr Gomes said human therapies were a distant prospect but: "From what we know so far we don't think you'd have to take it from 20 years until we die.
"It seems we can start when we're already old, but not too old that we're already damaged.
"If started at 40 you would probably have a much nicer window of health ageing - but I would guess that, we have to do clinical trials."
Prof Tim Spector, from Kings College London, commented: "This is an intriguing and exciting finding that some aspects of the ageing process are reversible.
"It is however a long and tough way to go from these nice mouse experiments to showing real anti-ageing effects in humans without side effects."
Dr Ali Tavassoli, from the University of Southampton argued: "It is important to note, that they did not see any changes in the mouse itself.
"This could be for one of two reasons. Either they need to treat for longer so that the changes occurring in the cells have time to affect the whole organism, or alternatively, the biochemical changes by themselves are not sufficient to reverse the physical changes associated with ageing in the mouse.
"More experiments are necessary to see which of these cases are true."