I think you've been unlucky. In my games, sometime the male is older, sometimes the female. A few times they've been the same age. I've also had the younger peep die first a few times as well.

On the transition to the next generation, the kids' ages are randomised so that they are close to the age of the original adoptee when they took over. That's intentional; the youngest may be aged several years and the oldest may be made younger. That way age isn't much of a factor when you decide who is to inherit the house.