Not just conceiving; _having_ a baby becomes much harder as you age. Birthing is a pretty physically stressful process for lots of women, and the following few years is a lot of very hard work. The most demanding of your life, probably.

Edit: Wanted to add, though, that _dating_ in your 30s and 40s just gets better and better. Don't worry about that. Maturity is a bonus, not a drawback.