I'm not 40 but I've found that after just having one kid it's made me MUCH more prone to gaining weight (and holding onto that weight even while I lose!) in my stomach.
Also, it's pretty normal for me within a small weight range (like the 5lbs you mentioned) to not lose anything from certain parts of my body. For instance, for awhile I was losing a lot from my bust/wasit/hips. Then that stopped for a good 5-10lbs loss where I was losing from my arms/thighs/neck. Now it appears that I might be losing from my waist/hips again.
It's not something to get discouraged over. I know it's annoying, it would be WONDERFUL to spot reduce but, unfortunately, our bodies get to pick where we lose the weight and we have no say in it. That being said we DO have a say in if we continue to lose weight or not and that will eventually help to cut back on your stomach fat.