Women aren't the only ones with a biological clock. New studies show that men have one too — and it may slow down early, around age 24. As with women who conceive past their peak, older men also increase the odds of having a child with an abnormality like autism or Down syndrome. One study of couples undergoing fertility treatments found that the father's age had as big an effect on conception and miscarriage as the mother's. Another study showed that men older than 40 were more likely to have a child with bipolar disorder.