"Because in every animal that walks upright, the deficiency of the fluids that fill the muscles appears first in the highest part: The face first grows lank and wrinkled; then the neck; then the breast and arms; the lower parts continuing to the last as plump as ever. So that covering all above with a basket, and regarding only what is below the girdle, it is impossible of two women to tell an old from a young one. And as in the dark all cats are grey, the pleasure of corporeal enjoyment with an old woman is at least equal, and frequently superior, every knack being by practice capable of improvement." - Ben Franklin, 1745
TL;DR - A woman with a juicy ass who's good in the sack will keep that juicy ass and get better in the sack while she gets older, and mature women rule.