In mice, it was activating telomerase but in a way more similar to how the process would be applied in humans. We use some vehicles (which can be viruses or nanomolecules) capable of introducing genes into the organism, the famous gene therapy. When treated with telomerase, middle-aged mice, we are able to delay aging and, thus, the diseases associated with it.
Telomerase is the antidote that nature has to realarge telomeres. It is not present in the adult organism, but is activated in the embryonic development and resets the telomeres of the new individual. It is interesting that cancer cells are the only immortals that exist, and that is because they have telomerase and keep their telomeres long.
Telomeres are structures that protect our genetic material. They are essential for the DNA to be intact. The simile of the plastic at the end of the shoe lace has been made. What happens is that they are shortened as our cells multiply throughout life to grow and regenerate tissues. And this is thought to be one of the causes of why we grow old and mortal.