Key message Tree structure equations derived from pipe model theory (PMT) are well-suited to estimate biomass allocation in Scots pine ( Pinus sylvestris L.) and Norway spruce ( Picea abies L. Karst.). However, age dependence of parameters should be accounted for when applying the equations. Context Pipe model theory-based (PMT-based) structure equations have been incorporated in many process-based models. However, more data concerning old-growth trees is needed to test the reliability and generality of the structure equations. Aims This study (1) tested the age independence of the PMT-based structure equations and (2) provided general information about the stability of tree structure with age. Methods A total of 162 Scots pine and 163 Norway spruce trees in four age groups were analysed to test the age effect on the parameters of structure equations using a linear mixed model. Biomass of stem, branch and foliage was estimated from destructive measurements, and with other tree dimensions, they were used to present the tree growth patterns. Results (1) Stem biomass proportion increased with age, while branch and foliage biomass proportion decreased; biomass allocation and most tree variables became steady after maturing. (2) PMT-based structure equations were well-suited to Scots pine and Norway spruce in all age groups; however, age dependence was detected in the parameters of these equations, except for the branch-related equations in Scots pine and stem form coefficient below the crown base in both species. Conclusion Our study (1) provides information applicable to predictions of growth and biomass allocation in old boreal stands and (2) suggests taking age effect into account when structure equations are implemented in forest growth models.