If you find a good mother maybe at the end of the year you can take 6 cuttings or so and take the heartiest one to replace the old mom. That way you have a fresh plant that isn't too mangled from clippings yet... I have seen bonsai moms that were almost 10 years old though. I dont think there is any advantage to keeping them around that long unless they are primo genetics you can get anywhere else. But I know that cells can only divide so many times before they start to get tired and old. Just as after 80 years on this planet our cells start to break down as well.