The oldest known tree that has been dated by counting tree rings is a Bristlecone Pine that has been named Methuselah. As of this year, it is 4841 years old. Another ancient Bristlecone Pine, called Prometheus was cut down in 1964. At that time, it was originally dated at 4840 years, but since the tree was cut above the base of the tree, further study increased that to 4862 years. If it was still alive today, it would be 4886 years old.
Bristlecone Pines are adapted for life just at the tree line on mountains. Their wood is very dense and resinous, making it very resistant to insects and disease. Living at high altitude, they grow very slowly, which helps them live very long lives without getting too big to keep standing.