Well, in theory there's no reason for a tree to die of old age, I think.
Not that I'm really an expert in the field, but my understanding is that trees start withering and die once they become too big, as the roots end up no longer being capable of keeping it alive as the foliage/mass/roots ratio becomes unbalanced, and/or the roots would have sucked all the nutrients from the patch of land they grow in/on.
That's one of the reasons why root pruning and refreshing soil is such an important part of bonsai maintenance: our trees are kept small and their roots thin and efficient, hence keeping the plant in balance.
That is: if kept in proper care for its entire life, a bonsai would largely outrun its siblings in the wild in terms of longevity...
Am I wrong?