DNA is the instruction-set from which the human body emerges. When a cell splits into two new cells, the DNA in its nucleus is replicated.
There are more than 3,000,000,000 "letters" in the human genome (what we call the set of DNA used to define a human), split into 23 chromosomes in the cell nucleus, and one small set within the cell's mitochondria (energy generator).
Replication is handled by a small set of machines called DNA polymerases. They're incredibly accurate, making only one mistake in about 10,000,000 copies. That means that there are about 300 mistakes per cell replication. However, there are also some post-processing machines which look out for these errors and correct them, which reduces the number of errors by 99%. This means that every time a cell replicates itself, there are possibly about 3 errors in the replication. We call those errors "mutations".
If a cell that had a mutation during its creation goes on to split itself into further cells, then those mutations will be carried forward as well. This means that the older a body is, and the more often that cells have split, the more mutations there are in that body.
A lot of the functions in DNA are copied in multiple places, so a broken line or two doesn't mean that the copy is a failure. Think of it like the population of a city composed of millions of people that all do different jobs. If a few of them fail to do their job, the city continues to work on.
Sometimes, the errors can cause issues where the new cells misbehave so badly that they affect the overall organism. Cancer is an example of this.
Because mutations increase throughout the life-time, an older person's body does not work as efficiently as a younger person's (it is more likely that a mutation is bad, than that it is good).
A sci-fi solution to this problem might be to have a periodic cleanse where all the cells of the body are inspected, and any DNA that differs from the average is corrected. However, that is (currently!) impossible.
In the meantime, we can help to reduce the chance that errors occur in the replication, which helps the body keep the mutations down as the body ages.
Professors David Sinclair and Lindsay Wu produced research recently showing that replication of DNA can be helped by activating enzymes called SIRT1, SIRT2, ... SIRT7. They discovered that a simple way to do this is to boost the amount of NAD+ contained in the cells.
NAD+ is a protein that is very abundant in young people's cells, but less abundant in older people's. It helps the cell catalyze ATP to power the various parts of the cell.
Professor Sinclair was previously known for his work identifying the effects of Resveratrol (which is found in red wine) on the SIRT1 enzyme. The NAD+ work is an expansion on that research.
In lab tests, it was found that by increasing NAD+ in cells, it was possible to boost the life-spans of mice by 20%. That's the equivalent of about 20 years in humans.
NAD+ doesn't survive outside cells, so it's not really feasible to give it in pill form. But, it was found that if you add NMN (nicotinamide mononucleotide) to water, then this is converted to NAD+ by the body's cells.
Human clinical trials are still under-way, but in the meantime, Wu and Sinclair are so confident in this that they take NMN supplements themselves.