On the life expectancy of trees:
Most trees do not die at roughly 150 years of age. For most trees, in fact, age 150 is a time of steady growth and phenomenal seed production, equivalent to the late teens in humans. Most tree species have natural lifespans of about 400-500 years, and there are a good number of species which live thousands of years. Bristlecone pines, redwoods, sequoias, limber pines and Ponderosa pines all have life expectancies of over 1000 years. Even the common oak frequently lives to celebrate its millennial birthday. As the saying goes, “Oak takes 300 years to grow, 300 years it stays, 300 years it takes to decline.”
There are some yews in Scotland that may be as old as the Methuselah bristlecone pine. It is impossible to tell their exact age, as yews do not always form annual rings like most tree species, and the centres of the older specimens are hollow. Mitchell’s Rule, which states that the girth of most trees grows ½" - 1" per year, can be used to estimate most trees’ age, but yews are a notable exception to Mitchell’s Rule. According to most estimations, yews grow an average of 5 mm per year in youth, slowing to 3 mm in later years. Some yews are also known to have stopped growing for up to 300 years at a time. (Don’t go chopping down that old yew just because it hasn’t grown in years and it sounds a little hollow - this is perfectly normal yew behaviour.)
Based on estimated annual growth of 3-5 mm per year, and allowing for episodes of zero growth, the Fortingall Yew is estimated to be roughly 5000 years old. This is, of course, an extremely rough approximation, and a disputed one at that, but the estimate of Methuselah’s age is also an educated guess. (As far as I know, that age is estimated to be 4600 years, but it’s possible that new findings have superseded my information.)
Source: THOMAS, P. (2000). Trees: Their Natural History. Cambridge University Press, Cambridge.