Resumo. Knowledge of the demarcation of juvenile and mature wood is essential both for wood technology and forest management. Thinning, in most cases, must take into account this parameter, as the juvenile wood has inferior characteristics compared to mature wood, and different proportions of each type of wood will affect the quality and the use of the wood. Thus, this study aimed to determine the age of maturation of the wood using anatomical characteristics of Carya illinoinensis (Wangenh) K. Koch wood, through the segregation of juvenile and mature wood. Three adult trees, of good trunk, with diameter greater than 30 cm DBH, were chosen from the region Encosta Superior do Nordeste in the State of the Rio Grande do Sul. From each tree, one disc was removed, with approximately 2 cm in thickness, at 0.10 m in height of the trunk. From each disc was removed a central portion of 2 cm in width, well oriented in the radial direction, including the pith in the center, and this central portion was divided into samples "A" and "B". One of these two samples was selected for separation of the initial wood of each growth ring for maceration (Jeffrey method ). First, the length, width and diameter of the lumen of one hundred fibers were measured in the first ring (next to the bark) and later, thirty fibers were defined per growth ring as statistically sufficient for the assay. The thickness of the walls of the fiber was taken as the half of the difference of the diameter of the fiber and the lumen. The segregation of the two types of wood was defined by the radial variation (pith-bark) of the anatomical characteristics (length, diameter, width of the lumen and thickness of the wall of fibers), through two simple linear regressions. The results indicate that the fiber length is the best characteristic for the definition of the year of segregation. The age of segregation of juvenile-mature wood was defined as 16 years. In their turn, the anatomical parameters, diameter of fibers, width of the lumen and thickness of the wall of fibers were shown to be inadequate for estimating the age of segregation.