Astragalus Membranaceus is a herb that has traditionally been used in a wide variety of herbal blends and 'natural' remedies in Asiatic medicine, such as Dang-gui buxue tang (Astragalus paired with Angelicae Sinensis; also known as DBT). It has recently been investigated more into due to its heart healthy (cardioprotective) effects, its anti-inflammatory effects, and the potential for Astragalus extract to enhance longevity and lifespan. Interestingly, the traditional chinese preparation method of DBT was shown to be a very effective way of extracting the main ingredients from the plant.
There does actually appear to be a ton of synergism between Astragalus Membranceus and Angelicae Sinensis, so it may be prudent to use the latter if deciding to get the former.
It has a variety of active ingredients, and the 'main' component tends to be seen as Astragaloside IV (which has been extracted, isolated, and patented as TA-65; marketed as a life enhancer). However, the bioavailability of this compound is fairly low and not much may get into circulation. Any effect attributed to Astragaloside IV would have to be one at low concentrations, which may be it's cardioprotective effects.
The flavonoid content may also contribute to cardioprotection, as flavonoids tend to possess this mechanism, and the polysacccharide content can also aid this by being a potent anti-inflammatory agent and reducing cholesterol levels in a manner similar to psyllium husk (a fiber).
Astragaloside IV, as well as a putative component only known as HDTIC (two isomers) have been shown to actually reduce the phenotype associated with aging. High dose Astragaloside IV orally (via TA-65) can reduce metabolic complications of aging. However, studies on TA-65 are problematic as they are all from persons who can profit. HDTIC is currently in preliminary stages, and although it shows promise in reversing or attenuating aging of skin cells it has only one rat study to date with topical administration.
No studies have, at this time, shown Astragalus to enhance the length of life. It can indeed preserve telomere length, but this does not seem to enhance the lifespan in the one study that measured both.
Astragalus Membranceus appears to be quite a healthy compound for longevity but may not actually promote longer life. It, like Resveratrol, appears to be a compound that gives life to your years rather than years to your life.