Background The debate over whether beginning a sport early and specializing in it promotes athletic success in adulthood has been the subject of academic discussion for decades. Current evidence suggests the opposite. Multisport participation during youth and a later start in the primary sport may positively influence athletic success in adulthood, although the effects on youth athletes remain inconclusive. However, youth sports performance and developmental potential are central to success in talent selection processes, which decisively shape athletes` subsequent talent development. Therefore, this study aimed to determine the handball training age and prevalence rates of sport specialization in talented female U15 and male U16 handball players and to examine their associations with the attained selection tier. In addition, the years of practice in other sports (i.e., depth), the number of other sports (i.e., breadth), and the type of other sports were compared between selection tiers. Results The vast majority of female (92.8%) and male (97.0%) players were classified as specialized athletes. Almost one third of the female (30.9%) and male players (30.8%) have never been involved in sports other than handball (i.e., single-sport players). Multisport engagement was not significantly associated with the selection tier in both sexes. Furthermore, there were no adverse associations between the breadth, depth, and type of other sports practiced and the selection tier. Female (g = 0.59) and male (g = 0.40) players who achieved higher selection tiers demonstrated a higher training age compared to non-selected players. Conclusions In light of the players` mid-teen age, the high prevalence rates of sport specialization were as expected. Nevertheless, the relatively high proportion of single-sport players may give cause for concern, particularly given that multisport engagement did not impede players from reaching high selection tiers. In fact, multisport engagement can be considered a viable alternative developmental pathway toward adult high performance. Furthermore, training age was associated with selection tier. Consequently, having a high training age may increase the likelihood of being selected as a "talent" which should be considered in talent selection processes.
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