Right, the question is why would stopping growth necessarily limit an organism's lifespan. I don't think that's a given. If you don't age and can stop growing

Right, the question is why would stopping growth necessarily limit an organism's lifespan. I don't think that's a given. If you don't age and can stop growing at optimum size, then that's potentially a huge evolutionary advantage. And since even “marginally better” can often be selected given enough pressure, it's hard to imagine why growing older is so prevalent.