I would agree that it makes sense to raise the age limit on crones. There is a definitive difference between old age, and degradation. In many mythologies, the old wizards, witches, crones, what have you are old - or even ancient. However, through the use of their magics, they are incredibly hearty and deceptively strong, with bodies that may seem shriveled and dying, but are still more capable than yours or mine.
It would seem that the point of the whole thing is that in this specific case especially, the nature magic was keeping their bodies from withering and decaying. It seems a bit silly in fact, to have a mostly nature based nation where the entire idea behind their mages is that they are very long lived because of their connection to nature, but still, they are on their last legs.
I'm not saying it can't make sense either way, simply that since it CAN make sense either way, it's a viable area to remove one of the numerous weaknesses that plague the nation.