As someone that’s been involved with clinical trials there are many reasons to target different age groups. The first is what age group is most at risk adults verse children. Second is that even when equal the ability to enroll and follow adults is significantly easier than enrolling and following children. An adult answers for themself, self travel, etc… Completely opposite for younger age groups getting harder and harder as the child gets younger. There’s a lot more drop out, not good follow up, harder to measure side effects and so forth. Finally, the age groups themselves are somewhat epidemiological relics that usually are dependent on number of age cats someone designated, can do 65+, 18-65, under 18. Sometimes you let the disease and outcomes dictate age cats like 0-5, 5-18, 18-65, 65+ for worse outcomes for young and old. Sometimes like covid went 18+ for all adults 12-18 for puberty range, 5-11 for children, and 0-5 for young children, it’s pretty standard.