They can not and try not to wash it off. You may be trying to go natural but are putting toxins or poisons into your system. Many insecticides are extremely dangerous. 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.