What Is Ben Franklin's Letter on Choosing a Mistress?

Alie Ward says Ben Franklin's 1745 letter to a friend, now titled advice to a friend on choosing a mistress, tells him to prefer old women to young ones.

The letter gives eight reasons. Older women have more knowledge of the world, minds better stored with observation, and conversation that is more improving and more lastingly agreeable. When women cease to be handsome, they study to be good, and there is hardly an old woman who is not a good woman. Taking an old lady as a mistress means pregnancy does not come as easily, and a visit looks less suspicious because it can look like keeping the elderly company. In every animal that walks upright, the deficiency of the fluids appears first in the highest part: the face grows lank and wrinkled, then the neck, then the breast and arms, while the lower parts stay as plump as ever. Cover all above with a basket and regard only what is below the girdle, and it is impossible to know an old woman from a young one. As in the dark, all cats are gray. The pleasure of corporal enjoyment with an old woman is at least equal and frequently superior because older women are more practiced. The final reason is that they are grateful.