Volatile oils are lost if the herbs is dried too quickly or at too high of temperatures. Mold and Mildew can develop if the herb is dried too slowly or incompletely. Most plants have different varieties. Each variety has slightly different levels of nutrients. The variety which has the higher amount of active ingredient is more potent.
Aged garlic is basically dead garlic, The beneficial part of garlic is allicin.
Allicin is the result of two chemicals mixed. If you wait until the level of allicin has diminished you have also waited until the potential for benefit is lessened. This article is only about herbs there is another whole article about Vitamins and Minerals and whether they are synthetic or natural , or chelated to the correct medium, etc. If you get a bottle of herbs dirt cheap it is probably only worth its weight in dirt. That is bad enough but it may also contain things that will hurt rather than help your health.