The formation of future athletes must be structured and planned in order to achieve the highest performance level. It is necessary to take into account the rates of development and maturation of young athletes in this early period in order to adjust the training loads to their developmental stages. The aim of this study has been to analyze the morphological age in young swimmers and triathletes to establish differences between their chronological age, groups and gender. Applying the anthropometric method and equations of Siret et al., we determined the biological maturation in 37 young elite athletes. The results showed that most athletes are in an advanced state of development (70.8%) compared to their chronological age. In swimmers this rate of development was more noticeable than in triathletes. If athletes have an advanced morphological age compared to chronological age, these young athletes could obtain better results in competition and enter elite programs more easily. The modified morphological age index (IDCm, from Spanish) is presented like a reliable and non-invasive method to establish the maturation state when we select potential sport talent or when we adapt training loads to the developmental state of athletes.Análisis de la edad morfológica en nadadores y triatletas adolescentes