Interesting thread but I firmly believe that love has no barrier as long as the concerned people involved in a relationship both mutually agree on it, love each other and are happy with the way things are going on. If both of them are happy, and both are adults, then there shouldn't be any issues.
Age is not something I would personally consider while seeking a relationship with a person. I know some who are in their 40's and the other half is in their 20's and they have been living happily for many years now. I don't find it as a hindrance as long as you love each other (which is what is most important in these aspects).
I agree with you HarryMan however there is the issue of children to consider. If the man is younger by 20 years, he either must have no interest in having children, be okay with helping raise his partner's children....or may have children of his own. If a middle-aged woman is fortunate enough to find herself in that situation then what you say stands. Easier said than done, however. There is no question that it works more often when the man is the older of the two.