"New Scientist Magazine reports on findings that suggest that delaying fatherhood may increase the risk of fathering children with disorders such as Apert syndrome, Autism and Schizophrenia. The article reports that 'although there is a big increase in risk for many disorders, it's a big increase in a very small risk. A 40-year-old is about 50 per cent more likely to father an autistic child than a 20-year-old is, for instance, but the overall risk is only about 1 per cent to start with.' In other words: time to start mating before those tadpoles turn into toads."
Voice of experience here, having disabled kids is the least of your worries if you're going to be an older parent. I had my first kid when I was 35 and my ex-wife was 30. My oldest daughter's autistic. Younger daughter is two years younger and gifted (takes after her old man).
When you're 20 it's not all that hard to get up at 3:00 AM to feed and diaper the baby, hell, when I was 20 I'd drink 'til 3 and show up at work bright and bushy tailed. But at 35 it's hell. It would be worse at 40. By the time my kids were grown I was over 50. My daughters are delaying their parenthood as well, so it's unlikely I'll ever meet any grandchildren I may have, and if I live long enough to they won't remember me.
Have 'em young. Just don't have more than two.