That was the biggest thing to me, the size of the tree and their age. The picture I get is that the mean age of the older trees is maybe 70 years, after which they are all harvested. This excluding the good stock tree here and there that is left to replenish the forest, etc.
Left to go forward and age further, the properties of a tree do change. The wood compresses and condenses both with age and as a result of the stress of the tree blowing in the wind, and supporting its own weight. The difference in the wood characteristics between a 70 year old stem and a 120 year one are significant.
I always wondered what this difference was between "Virgin" and second growth. TO me, all I can reason is that it is a matter of age.
This upholds my suspicion that, in the mountains of Bern, the trees we can find of suitable age -some maybe 150 years or even more- would have that property.
For building log structures, they want those big, old trees. The wood is better, the material stronger and more stable, and more durable. Also, the greater width of the timber gives the structure a more impressive and stronger look.
In the United States, I see a turnaround in forest management of 70 years like you say.