Resveratrol and quercetin are both made by plants and are present in red wine, fruits and vegetables in varying amounts; they are naturally occuring compounds that have historically been part of a healthy diet. Animal studies show that they stimulate metabolism in ways that promote longevity, increase energy, help keep weight off when you lose it, reduce allergies, and reduce the risk of infections, cancer, heart attacks, and strokes. Studies of human populations show increased longevity where natural diets are higher in these compounds.
Animal studies tend to use very high doses compared to what is typically found in a normal diet. Given my conservative tendencies with supplements, I've been taking a relatively low dose, something that puts me on the high end of a natural diet.
Even at relatively low dosage compared to animal experiments, I can see a clear positive effect. I am a classic pre-Syndrome X individual, with an apple-shaped middle (before I lost weight), increasing insulin resistance and pre-diabetic glucose profile, and relatively high trigylcerides. Before I started taking resveratrol and quercetin, I reduced my carbohydrate intake, switched to a low-glycemic-index diet, added magnesium and zinc supplements, increased omega-3's, and increased excercise levels. All of these changes helped, but after I started taking resveratrol and quercetin, my insulin/glucose metabolism was dramatically better, and I could eat more carbohydrate at a sitting without seeing a spike up in blood sugar. These supplements basically took me from a pre-diabetic condition to high-end-of-normal, which is nice to see.
The great thing about taking supplements is that you just have to pop a few capsules or take a tablespoon or two of fish/flax oil. This is a very minor adjustment to a lifestyle, but gives very significant benefits when you take omega-3's and resveratrol/quercetin. More radical changes are also good, but if they are too difficult to undertake or sustain, do make sure you the easy steps. You will notice the difference.
Age considerations: we don't know how these supplements affect development, so while they should be OK at all ages, I would recommend starting them at age 25 or so.