This is the first time a living organ has been 'reverse aged', and the breakthrough, made by scientists from the University Edinburgh, has BIG implications for regenerative medicine in humans.
The revamped organ was the thymus, which sits near the heart and plays a critical role in immune function, but becomes smaller and less effective with age.
The scientists used a drug to increase activity of a gene called Foxn1, which naturally gets shut down as the thymus ages.
Their results, published in Development, revealed this caused the thymus in elderly mice to regenerate and start functioning like a much-younger organ.
Dr Nick Bredenkamp told the BBC: "The exciting thing really is the manner in which it is done. We've targeted a single gene and we've been able to regenerate an entire organ."
While the process could eventually be adapted to work in humans, the scientists argue that it would need to be very tightly controlled to ensure the immune system did not go into overdrive.
Most excitingly, the work raises the possibility that we could one day regenerate organs such as the heart or brain by targeting certain genes.