Dense Yew comes from real slow growth due to growing conditions, nothing to do with age, The warbows of old were made from High Altitude yew from Europe, this is a different species of yew tree from the yew that grows in Oregon, European is Taxus Baccata, American Yew is Taxus Brefiola, They are quite different, the European yew is much heavier physically than the American, also the European yew is also much stronger and will handle much more ill treatment, The average for high altitude italian yew is 50-60 rings to the inch, some are 80 rings and i have had some at 100 rings to the inch. The quality has dropped since the middle ages due to the fact that this wood is no longer farmed and tended like it was, people tended the trees and removed unwanted branches etc, for the life of the tree until it was cut for bow wood.
hope this helps
steve