This might be one of the main reasons why some people in the age range of 40 or above supplement. Mainly, supplementing for 'aging' include two goals:
Improve the quality of life and 'youthfulness' of their life
Reverse the external and internal 'symptoms' of aging
The primary goal should be the latter. However, sadly there is no simple way to determine what a 'symptom' of aging is.
We might see externally that our skin wrinkles, we lose hair, or that we become more susceptible to diseases. However, you cannot supplement a wrinkle; one needs to see what caused the wrinkling (ie. enzyme X), and then see what is wrong with enzyme X (compound Y), and then keep on working back until we see something which changes due to nothing but age.
The page for Alpha-Lipoic Acid, a common anti-aging supplement, demonstrates two of these mechanisms. One is a reduction of glutathione levels (an anti-oxidant system) due to perturbations on the genetic level (which are still not fully understood). All we know is that ALA can reverse these adverse changes. Another change in aging is reductions in vitamin C content of the heart and liver, which is also reversed with ALA supplementation.
So there is no rhyme or reason with anti-aging supplements. We just find out a cause of aging, how to reverse it, and denote that compound anti-aging.
Others, like Pyrolloquinoline Quinone, are marketed as anti-aging supplements despite no real evidence to reverse markers of aging. They just increase quality of life and vitality (goal 1).
Overall though, there is no assurance that these compounds do increase lifespan. That would require human trials ranging over 100 years long, and everyone reading this would be dead by then. The best one can do is look at changes that occur, are correlated with the adverse effects of aging, and then fight against them with a healthy, active lifestyle and proper supplementation.