This is only an issue for women. Risk of problems with pregnancy (and birth defects) goes up significantly for women as they get older (best time to have kids for women is in their 20's). The male reproductive system is much, much simpler though and I don't think age is a factor for men at all. Of course as you age you'd have to find someone increasingly younger than you to carry a child. That would be the only problem.

On the other hand some men would not consider that to be a problem at all (assuming you can find a mate). And if you're a 40 yearold millionaire you shouldn't have a problem finding a 20-something to have a kid with. :-)