Human beings are born pretty helpless, with a lot of developing to do. And just as you must learn such skills as how to walk, so must your immune system learn to defend against infections. As time passes, your immune system matures through different stages, much the way you advanced from crawling to standing, walking and running.
This process is one of the reasons scientists study the immune response to a vaccine in different age groups, and why, for example, the COVID-19 vaccines need to be tested separately in children ages 5-11 and those 12-16. Doctors want to use the vaccine dose that provides the best protection with the fewest side effects. And that’s going to depend on how the immune system is working based on how developed it is – something you can’t really tell from the outside.