Many trees survive centuries and die only in disasters like storms and lightnings or infections. So do they have no set life span? Are they exceptionally resistant to genetic mutations and have very less cell cycle regulations which keep them 'forever young'?
There are certain annual(eg.peas) and biennial plants(eg.carrot) also which die at fixed times.
But for perennial plants :
Trees endure as long as they do basically because they're nonhierarchical organisms. In animals, all vital functions are controlled by the central nervous system, the guiding element of which is the brain. When the brain dies, so does the animal. By contrast, vital functions in trees are decentralized. A large part of the tree can die, and indeed routinely does die, without killing off the tree as a whole. Most of a mature tree is dead except for a few layers under the bark.....
All trees die eventually, of course. Four thousand years is old compared to the life spans of gossamer creatures like ourselves, but in the context of geologic time it's the blink of an eye. As they get older trees become more susceptible to disease, pests, and other perils, and inevitably these take their toll.