That’s also true for peanut butter too, in many cases.
Sufficient clinical trials haven’t been completed for the younger age group. It was necessary to run the initial trial and observe the results in people of consenting age first just in case there were problems.
A long term is the only thing that can lead to that knowledge, unfortunately.
Many people will opt for taking the vaccine despite this, in the hopes of getting past many restrictions like not being able to visit precious family members that live in other households, or towns, or provinces, or countries. They will opt for it based on their personal risk versus reward assessment. Everyone is free to make their own assessment and act accordingly.
For me personally, I don’t see it as a choice between taking a vaccine, or not taking a vaccine. Rather, I see it as a choice between:
1) taking a new vaccine
or
2) risking infection from a potentially deadly virus each time I shop or do other essential things, and not being able to do many non-essential things I would really like to do again such as socializing with relatives and friends in person, or eating out at a restaurant with delicious food, or traveling.