Can't say that I can agree. 5 years would be absolutely no problem. Other than the obvious physical attractiveness, I can see no reason why anyone would seriously want someone 20 years younger. Although I'm sure to be deluged with reponses that start with, "My grandmother..." inter-generational dating is IMHO, almost always a mistake. What's tolerable at 30 and 50, is a nightmare at 50 and 70.
I'm 4 years younger than my wife, and we're doing fine after 20+ years. The important thing is that we're of the same time, and can relate to each other being in the same stage of life.
Well most 70 years olds are not in that bad shape, but....I do agree that 60 and 80 might be quite a gap. Even so, at that age the woman may appreciate not getting “bothered” and have plenty of time to go shopping to spend the guy’s $$. :^)