Aim: To substantiate the methodology for assessing the biological age of adolescent high schoolers (boys and girls). Materials and Methods: Boys aged 14-15 (n = 102) and girls aged 12-13 (n = 101) were studied. The choice of these age groups is explained by the presence of the most significant individual differences in morphological and functional characteristics in these adolescents. Methods: anthropometry, somatometry, somatoscopy, tonometry, spirometry, dynamometry, Stange’s test, Rufier and Robinson indices. The interaction of the results of different methods of assessing biological age was conducted on one sex-age sample using factor analysis (principal components method). Results: It has been established that the biological age of adolescents can be reliably determined using a set of three components: an assessment of the level of physical development, the index of heterochronicity, and the degree of manifestation of secondary sexual characteristics (for boys with a probability of 90.2 %, for girls – 92.1 %). The percentage ratio of these components has sexual peculiarities. Methodologies for assessing biological age for scientific purposes and rapid assessment for use in the practice of physical education and sports have been proposed. The indicators of rapid assessment are body length and the degree of hair development in the axilla. The reliability of the rapid assessment is 81.4 % for boys and 83.2 % for girls. Conclusions: Developing adequate physical activities for adolescents of different biological ages will help improve the physical health of the younger generation.