It's the age of the mother's eggs that really matter as far as genetic diseases. Ideally, if you're a woman who wants to have children one day but know you'll very possibly be in your late 30s, freezing eggs is a good option.
So is adoption but yeah most people look down on that. I think you have to suck at life to dislike children or to think there's ever a perfect situation to be a parent. When people try to plan what their family will be like in 10 years I just roll my eyes.