Growth rate determines wood quality. A tree that took two centuries to reach harvest lays down narrow annual rings, producing dense, uniform timber. Plantation stock reaching harvest in thirty years produces wide rings, a high proportion of low-density earlywood, and far more sapwood.
Heartwood is the decisive difference. As a tree ages, inner wood ceases to conduct sap and fills with extractives — natural compounds that are toxic to decay fungi. Old-growth boards are predominantly heartwood. Fast-growth boards frequently are not.
Grain orientation compounds the effect. Vertical grain, or quarter-sawn, stock has growth rings running roughly perpendicular to the face. It moves far less across its width with seasonal humidity swings, which means joints stay tight and finish films crack later.
The practical consequence is uncomfortable for the replacement industry: the wood already in your wall is usually superior to the wood that would replace it. A localized rot repair preserves that material. A replacement program discards it.