Just to chime in here, I personally know two 34+ year old moms who had children in the past 2 years with Down Syndrome, after having numerous miscarriages. It seems a lot of young geeky guys commenting here forget that it's not just men who have a child so if you want to have a kid in your 30s and 40s after building a career, it's going to be difficult to find a partner at that same age who has the same goals.
It's not just the age of the female partner that affects the child's health. As men age, the risk of passing genetic mutations (including those that cause diseases) onto their offspring increases. This article talks the study: http://www.telegraph.co.uk/health/healthiness/9492343/Men-ad...
So if you're an older guy, having kids with a younger woman doesn't make this problem go away.
