and I don't think one solid answer. But, what everyone agrees on is:
Unless there is some other factor effecting the ovaries, as you age there are less good ones in there. Does not mean that there aren't still good ones, just less than when you were twenty. It's all about finding the one good one and all the other factors being in place!
Other ladies will pipe in here but this is what I have been told and what I have gotten from a year on the board!