[throwup238]

It's not about the age of the individual trees but the overall forest since it was last disturbed by humans (on a large scale like logging or agriculture).

Age matters because there are several "thresholds" during which the diversity of species significantly increases and that starts with the older trees falling and decomposing. The conifers in the PNW take anywhere from 50-150 years to decompose before they start breaking apart and littering the forest floor. Once the first generation of trees is broken up and spread around by the wildlife and the fungi species are fully established, the growth of third/fourth/fifth generation of trees and plants become a lot more vigorous. When the second and third generation of trees start falling, they end up creating the dense habitats that supports large food webs from the rodents on up. All the while, the tree roots pull nutrients from deeper and deeper in the earth, allowing the rain and cold to build up a layer of peat on the forest floor that cycles and stores the nutrients, improving the quality of the dirt.

It just takes a while for all these processes to build up.