A menopause website says, "sex drive decreases gradually with age in both men and women, but women are two to three times more likely to be affected by a decline in sex drive as they age." The ages they mention are 40's and 50's.
Well, that fits my marriage perfectly. Good to know that my wife and I are not some freakish rarity. Sure there are many exceptions, but this statement clearly suggests that it's common for the woman to totally lose interest while being in a marriage where the man has not lost interest.