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The age of a Joshua Tree cannot be determined. The trunk of a Joshua tree is made of thousands of small fibers and lacks annual growth rings, making it difficult to determine the tree's age. As far as I can recollect, I think the oldest tree is, Sri Maha Bodhi, which is a Sacred Fig (Peepal) tree in Anuradhapura, Sri Lanka. It is said to be a sapling from the historical Bodhi tree under which Buddha became enlightened. It was planted in 288 BC, and is the oldest living human-planted tree in the world with a known planting date.