https://www.ldeming.com/longevityfaq
In the 1930s, investigators wanted to do an experiment to see if stunted growth rates during the Great Depression might impact lifespan. They tested this in rats by feeding them less food than they would normally eat. To their surprise, this actually made the rats live longer! This was a seminal discovery. For the first time, we changed the environment of an animal to make it live longer than it normally would.
Since then, investigators have tried to uncover how this works. The effect depends on what genes you have, what you are eating and how much less you eat. If you take many genetically distinct mouse strains and put them on the same diet (cutting calories by ~40%), sometimes fewer than 1/5 of the mouse strains live longer. Diet composition also plays a role. Just decreasing protein or a specific amino acid, while keeping total calorie intake the same, can result in a lifespan extension in mice. Feeding mice a ketogenic diet also seems to help. Decreasing food intake by too much will result in starvation, so finding a diet that works can depend on the situation.
In papers published in 1983-1993, investigators introduced the concept that a gene could control lifespan. I got my start in science when one of the founders of this field, Cynthia Kenyon, agreed to let me work in her lab as a 12 year old kid. I'll always be grateful for her kindness and mentorship. Previously we'd known that caloric restriction could make animals live longer, but Kenyon and other scientists including Michael Klass, David Friedman and Tom Johnson found mutant genes that could make worms live longer. The gene that Kenyon found encoded a protein that is similar to insulin-like growth factor and insulin receptors in humans. In mice, mutating members of both of those pathways can increase lifespan. One of the longest-lived mouse mutants we have today is a dwarf mouse. In one study, people with similar dwarf mutations seemed to suffer less age-related disease than their non-mutated relatives.
Dracula wanted to drink young blood, but what does that have to do with aging? A paper published in the 70's showed that linking old and young female mice so that they share a bloodstream increased lifespan. Decades later, in 2005, scientists at Stanford showed that this procedure might help old muscle stem cells repair wounds. Then, in 2011, a succession of papers came out showing that this procedure and others like it (such as injecting young blood into old mice) made mice better at remembering things, and improved heart and muscle function with age. These discoveries increased excitement and interest in the field, and lead to a wave of startups.
You may have heard mitochondria referred to as the 'powerhouses' of the cell. It's funny, they do literally run like a dam generating hydroelectric power! - They pump protons (positively charged particles) one way, then use them as they slide back to run a kind of motor that makes a small energetic molecule used by many entities in the cell. One concept that comes up when people talk about mitochondria is 'oxidative stress' - the idea that if molecules are very reactive (say they have oxygen, acquire some extra electrons, and now want to discharge them onto other molecules), they are likely to interfere with a lot of other molecules in the cell that should be left to their own devices.
Weirdly, the story has turned on its head over time. It's true that it is bad to pump an animal full of reactive oxygen species, and that you can make a mouse live longer by increasing the level of proteins that are supposed to clean up mitochondria. But you can also mutate things that should be helping the mitochondria, and end up increasing lifespan! It's counterintuitive, and one hypothesis is that a little bit of stress is good because it forces your cells to put up their defenses and ramp up production of molecules that neuter the reactive oxygen species. But we don't really know.
https://static1.squarespace.com/static/5a2069c3ccc5c5325fd05b02/t/5a474c6441920273ab555830/1514622053609/2005_schriner.pdf
https://static1.squarespace.com/static/5a2069c3ccc5c5325fd05b02/t/5a4747b49140b7ea14d49c9a/1514620854382/2011_guevara-aguirre.pdf