Yes, it is for the same species.

I have old growth red pine and modern pine in my property, the old growth stuff is so much heavier and stronger it's comical.

As said in the OP, it's largely because the old trees are much older, so the wood is simply denser.

I don’t believe they are. I don’t believe it because I’ve removed “new” Douglas fir trees and enjoyed the tight grain lumber. The growing conditions and age are important, not when they were grown. Old growth is just old trees grown in natural forest conditions. That still happens all over Oregon.

Yes, growing conditions are certainly most important. Old growth had particular growing conditions - a very high and dense canopy for once.

Species is also more important FWIW! If you're making a window you should get a rot resistant species, not old growth stuff.

The point in OP is that if you find old growth stuff, hey, try to keep it because it's probably really good wood.