Purpose. Players’ tactical skills are crucial for a successful, high-standard career in soccer. Although recent studies propose and apply the analysis of young players’ tactical skills through the understanding of their knowledge about tactical principles, no research has compared the development of these specific tactical skills throughout a soccer specific season in different youth academies. This article aimed to compare the development of tactical skills in U-14 and U-15 soccer players throughout a season. Methods. The tactical skills of 30 athletes (16 U-15 and 14 U-14) of a sports club were evaluated during 10 months with the System of Tactical Assessment in Soccer (FUT-SAT). The incidence of tactical principles and the percentage of successful principles in offense and defence were compared with the use of a two-way ANOVA (category × time). Results. Reduction was observed in the incidence of actions of offensive unity in the U-15 athletes throughout the season and a higher incidence of offensive coverage compared with U-14. Moreover, U-15 athletes increased their percentage of successful actions, both offensive and defensive, and presented a higher performance than U-14 players. Conclusions. Tactical performance of U-15 soccer athletes increases over a sports season, which is not observed in the U-14 category, although players from both groups presented changes in the incidence of offensive and defensive tactical principles. This supports the non-linearity of tactical skills acquisition and development in soccer, as well as justifies the need of constant assessment during the training process across the ages.