Not necessarily. Although more and more women are "fit and fabulous at forty," ovarian function doesn't follow this path.
Women are born with all the eggs they will ever have--current research into ovarian stem cells aside--and the egg number declines as women age. The best quality eggs are released first, in the teenage years (yes, teenagers do conceive more easily). Egg quality, as well as quantity, declines from then on. Loss of eggs speeds up as women reach their mid-thirties. This is particularly bad news for women who have delayed child-bearing while pursuing their career goals or finding the right partner.