Ageing is a disease. Just as treatments have been found for other diseases, ageing can be slowed or reversed as well.
Not that long ago, scientists noticed that at the end of each DNA strand there were thousands of nucleotides. Nucleotides are used for genetic coding. That is to say, certain sequences of nucleotides instruct the body to synthesize certain proteins when the genetic code in the DNA is being read. DNA is really like a massive software program.
In any case, when scientists noticed all of this code at the end of a DNA strand, at first they couldn't figure out what is was for, so they thought it must be gibberish. Later they noticed that everytime a cell divided (e.g. everytime the DNA was read) a piece of this gibberish was lopped off. Eventually it became understood that this "gibberish" was the basis of aging.
Each lopped off piece was termed a telomere. Each telomere contains the same nucleotide sequence (code). There are about 15,000 telomere nucleotides at the end of the DNA strands in an embryo. This has been shortened to about 10,000 when you are born and when it reaches a length around 5,000, the body dies. It turns out that the telomeres play a role in stabilizing DNA strands during the copying process, when their length gets down to around 5,000 nucleotides the telomere length becomes to short to provide adequate stabilization and errors (genetic mutations) occur in the copying process and/or the cells become senescent (become inactive, go to sleep). Errors or sleep, either way, the body stops functioning efficiently.
This telomere shortening occurs during cell division in all human cells except for sperm and egg cells. Sperm and egg cells are immortal. There is an enzyme called telomerase, it is expressed only in the sperm and egg cells. After these cells divide a telomere is lopped off, but telomerase re-adds a telomere to the DNA in sperm and egg cells. Hence, their telomere length never shortens - they live forever.
All cells contain the code to make telomerase, but that code (or signaling) is turned off in all cells except for sperm and egg cells. A specific protein combines with a certain portion of DNA and that is all that is required to prevent telomerase expression in a cell.
If scientists could find a way to prevent that (repressor) protein from complexing with DNA, then telomerase expression could be turned on and that cell would then be immortal.
As you can imagine, this is a big area of research. Here is a link to a company that has been active in this area for a long time, and here is a link to a basic tutorial on the subject of telomeres. Poke around the web site, the videos and other tutorials are extremely interesting. Even aspects such as the social consequences of curing aging are discussed.
Enough progress has been made that you can buy products in the over-the-counter market that reduce telomere shortening, but they're very expensive at the present time.