I am curious how old the Douglas Fir on the picture could be? Please note a person is standing there on the left close to the tree.
Approximation is pretty much impossible without, as Ron mentions, growth rings. As an example, I recently bucked up a blown down Douglas Fir that was about 14" in diameter, so not that large. Noting that I'd never seen such tightly packed growth rings, I took the time to count. It was a 300+ year old tree. Looking at it's unremarkable height and diameter, one would likely guess about twenty years.