<p>Despite its importance in forestry and wood science, the conventional classification of juvenile and mature wood remains ambiguous. Juvenile wood is typically defined as the inner region of the trunk where wood properties change rapidly with cambial age, i.e., the number of growth rings from the pith. The surrounding mature wood is characterized by smaller variations in these properties. A major limitation of this traditional framework is that the distinction between juvenile and mature wood depends strongly on which specific properties are considered. Moreover, the relationship between the degree of property variability and tree maturation has not been clearly established. This study demonstrates that the age-dependent evolution of a density operator, computed from the near-infrared (NIR) spectral matrix at each cambial age, effectively represents the tree maturation process. The density operator encapsulates comprehensive information on both individual variability and the interrelationships among multiple wood properties. Four representative tree species with distinct anatomical features were investigated. Changes in the density operator indicate that the transition from juvenile to mature wood corresponds to a phase transition in the variability of wood properties. Tree maturation is thus interpreted as a shift in the xylem tissue state from disorder to order. The proposed approach is canonical—it is independent of the specific wood properties considered, incorporates information on individual variation, and can therefore be applied to a wide range of data sources beyond NIR spectral measurements.</p>