Lets see...
A crazy scientist guy studying something he barely understands, or the actual species in question.
My money is on the species in question. However, the zerg don't age because they mutate so fast and their cells replace often. What Iesha meant was, if there was no need for evolution, then yes the zerg would age. Don't forget, that sample Stetmann was studying was also growing and evolving while in its tank.