Yeah, so what if a guy can father children into his 60s. He also faces the very real possibility of not living to see his children graduate high school.
I, for one, am a cradle robber, though not horribly. I'm 24, he's 20. The way I figure it is that i'm cheating the odds that a man's average lifespan is shorter than a woman's. If he's younger it should even things out. (ha ha. I'm not that silly really.) also, a man is at his, shall we say... "prime" in his late teens to mid 20s, whereas a woman doesn't really hit her prime until her early to mid 30s. (this isn't just my word. There have been studies conducted.) when i'm 30, he will still be in his 20s. I've been congratulated on this line of reasoning, but it's not actually important to me.
All this being said, I think may/december romances can work if both parties are mature, responsible, and loving. It seems that in this situation it's not necessarily age that's the problem. There may be a host of other issues.
(side note to niceguy: women actually may lose their charm earlier, as you suggested. That's why the majority of the primetime sitcoms on the networks feature middle-aged, pot-bellied, balding husbands with hot, young, size 6 wives. I have nothing against those types of men in real life. I just resent that society has become so shallow that it's perfectly acceptable to allow men to "let themselves go" a bit, but women are old hat once they get a few wrinkles and some stretch marks. And the entertainment industry fosters this attitude. Men with gray hair are called distinguished. Graying women are called hags. I'm not a feminist by any means, I just think that beauty and human worth, whether male or female, are inner things and that somewhere all of this has been lost. Ah-hem.)