Community as a product is integrated into all GoodNotes 5 apps - as long as the app is kept up to date. As such, all users (regardless of age) have the ability to access Community in a limited sense - anyone with GoodNotes 5 is able to browse and view the study materials uploaded to Community.
However, with that being said - a lot of the Community features, such as study material uploading and downloading, is restricted to registered members of Community. In order to register to be a member of Community, you have to have an Apple ID and be over the age of 16.
As such, whilst you can get access to some Community features at any age, to get the full Community experience you need to be over 16 and have a registered account.