From a biological standpoint, it's kind of a big deal, I guess. Purely from a relationship perspective, not so much. If I were 30 and she was 36, or if I were 29 and she were 35, it'd be less of a big deal. But I'm almost 34, and she's turning 40 literally in a few weeks.
For instance, I usually date 26-29 year olds or that range. Let's suppose I were to marry a 26 year old today, and we had a kid in the next year. We could theoretically keep having kids for another 14 or 15 years if we wanted another one. Or we could easily wait 10 years to have a kid.
So, let's suppose I married this 39 year old chick today. She wants to have kids eventually (I know this, she's told me.) It'd pretty much be a do or die situation to have them right now. Obviously, lots of women have kids at 40, but fertility starts dropping off very fast. So I think about that kind of shit.