Studies on seasonal tree-ring xylem formation are very relevant, allowing assessing the influence of environmental factors on growth with high temporal resolution and the way of plant adaptation to current and expected climate changes. Still little is known how the age of trees can affect their seasonal growth rate and the duration of xlogenesis. Due to this fact, the goal of our study was to evaluate tree-ring seasonal formation of Scots pine (Pinus sylvestris L.) growing in the forest-steppe zone of Southern Siberia for trees of two age groups (30 and 95 years). The main tasks were 1) to obtain data on the duration of cambial activity, tracheids enlargement, cell-wall thickening and tracheids maturation, and also 2) to compare anatomical structure of the formed annual rings of trees of different ages. Sampling was performed weekly during the growing season of 2014 from April to September. The results of the study showed that the duration of different phases of xylem formation differed for each group by 1-2 weeks, and at the end of the growing season young trees formed a narrower ring with a smaller number of cells than old trees. The tracheids size of the fully formed ring of the two groups of trees remained almost identical, whereas the dynamics of the cell-wall thickness showed significant differences. The obtained data can be used next as input parameters of processed-based models that relate environment to wood formation, and will allow obtaining a biologically determined agreement between measured and simulated values of the annual increment and the anatomical structure of xylem.