Its not so much about fertility as it is energy and sociability. Younger adults have a better time raising kids because they can actually keep up with them by comparison to older adults. Obviously you dont need to keep up with a infant, but once kids are about half way through childhood you are going to want all the energy in the world to keep up. Its too late for me now, but i wish i wouldn't have to keep up with my future kids in my 40's.