Wood use for structural purposes has increased in the last decade in Spain. However, as a raw material, wood needs to comply with requirements that are not always present. Knowledge about the wood quality from the trees on the stand is essential for providing feedback to forest managers and for taking the required actions to obtain suitable silviculture treatments. Two of the main wood species used in construction in Spain, Pinus nigra and Pinus sylvestris, have been studied in order to determine the amount of juvenile wood, which has been identified as a harmful characteristic for its decrease in quality of the physical-mechanical properties of these species. Being relevant for the best quality of timber in the part along the bole where the logs are obtained, the distribution of juvenile wood at different heights and the effect of several silvicultural treatments have had on juvenile wood formation has been considered. The juvenile-mature wood boundary (transition year) was calculated through segmented linear mixed models employing as variables annual latewood density, obtained through micro X-ray densitometry, silvicultural practices, and a drought index. The results show how juvenile and mature wood is distributed along the bole and the proportion of juvenile wood. Its reduction according to the different thinning and pruning silvicultural practices is presented.