Pregnancy at a later age not only helps you keep the flab on, it also helps you gain the weight back (and more) immediately after losing it during the delivery and through the breastfeeding nights.
I have always thought, though, that babies make great exercise machines. They are human dumbbells that increase in weight whether you like it or not. You can literally grow muscles just by carting them and all their stuff around. Not to mention all the cleaning and picking up you have to do for the messes they make.
And then there's the running after them that you have to do. I think, between age 1 and 4 are possibly one of the longest, most physically trying times for a parent as a child zips around, unmindful of his safety. And the furniture/appliances or people he might break.
I just console myself that an active child, barring special needs, is a healthy child. And my son could just be the healthiest child of all.