I'm thinking it has a lot to do with health before conceiving, on the part of the mother and father. I turned 30 last year and like you am in NO rush for children. I never really bought into the whole birth defects thing at a certain age.
I think it's important to be healthy now, make sure once you are ready that you are feeding yourself proper nutrients and eating healthy foods, staying away from a lot of chemicals, and just living a positive wholistic lifestyle in general (as well as your mate or future mate). For some regular detoxes or cleanses may help as well.
Just a couple of thoughts, I'm sure other will chime in with other ideas
