Most of what we think of as the effects of aging occur at a cellular level, and are the result of an accumulation of damage over the cell's lifetime. That damage is primarily caused by "oxidative stress". You've probably heard of free oxygen radicals. They're created all the time in your body, and they wreak havoc on cells by reacting with proteins and other structures that are otherwise stable. Over time, this damage tends to pile up. The result, generally speaking, is aging.
You've heard of antioxidants, and you've probably heard that they're very good for you. The reason is that they help protect you against aging and certain diseases by preventing free oxygen radicals, or "reactive oxygen species" (ROS) from damaging your cells. Your body has some of its own antioxidant solutions, but most of the time you have more ROS than antioxidants, and that results in oxidative stress unless the cells are able to repair the damage faster than it accumulates.
Enter the ketogenic diet. There are several ways (see the section called "Antioxidant mechanisms") the diet lowers the production of ROS, which is extremely helpful in preventing the damage to begin with. But perhaps even more important is an effect I've talked about before and will talk about again: cellular autophagy.
"Autophagy" is Greek for "self eating", and in this context it describes the process of cells breaking down, metabolizing and replacing their own internal structures like proteins and organelles. This turnover helps keep oxidative damage from accumulating. That's important for protection against a lot of diseases, including cancer, atherosclerosis and Alzheimer's disease, but now we're here to talk about aging.
I can go a long way here by quoting from this article from Nature:
A common characteristic of all ageing cells is the accumulation of damaged proteins and organelles... [I]t is easy to infer that a gradual decrease in autophagic activity with age could play a major role in the functional deterioration of ageing organisms. Conversely, caloric restriction, the only intervention known to slow down ageing, seems to improve autophagy induction, possibly owing to lower levels of insulin, an autophagy inhibitor.
And this one put out by the Laboratories for the Biological Mechanisms of Aging at Harvard Medical School:
The dietary regimen of calorie restriction extends lifespan in diverse organisms ranging from yeast to mammals... but how calorie restriction works is not yet known. One of the changes associated with calorie restriction is a shift in the way in which energy is generated and used; that is, there is a shift from energy produced by glycolysis... towards energy produced by the breakdown of proteins and lipids.
Take note of the emphasis on caloric restriction. As this paper points out, "Calorie restriction and the ketogenic diet share two characteristics: reduced carbohydrate intake and a compensatory rise in ketone bodies." This statement omits, possibly because it goes without saying, that both diets also significantly reduce insulin levels. Insulin, as the first quote points out, inhibits autophagy.
The point is that the ketogenic diet has many, perhaps most, of the same effects as fasting and calorie restriction. The main difference, since they produce the same metabolic state (ketosis) is that the ketogenic diet doesn't resort to breaking down the proteins that make up your body, since they're abundant in the diet.
What this has to do with aging is hopefully clear by now. Autophagy is a significant anti-aging process that's inhibited by insulin, and insulin levels are kept very low on a ketogenic diet. This was well-understood by Dr. Atkins, whose low-carb weight loss program you've probably heard about. You may not have heard of his book on how the diet affects aging, which can be previewed here and purchased here, and which offers a much more in-depth but very readable treatment of the subject.
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