Thanks for your question!
This is an awesome suggestion. As of the moment, we can only change the permission of your current/future members manually. The ability to control the permissions of users based on their age on the community is not yet possible. I will definitely share this with the team and our team regularly checks on feedback from users to see what we can add in the pipeline for future improvement of the platform.
As of the moment, what you can control based on the age of your members is their ability to comment, post or engage with your community. You can do this via the Moderation app where you can set when users can engage with your community based on their age within your community. Here is a community post17 about Moderation app that'll be helpful to you.