When a patron category is used on the PatronSelfRegistrationDefaultCategory syspref, the "Age required" and "Upperage limit" are not taken into account during the self registration. For example, if I set age required to 15 years old, I can register with a birth date = today.
The test is done in members/memberentry.pl, it must be moved to a module. But then we will need to handle this error from other areas.
Just confirming that this behavior still exists in version 21.05
After I added my comment, I was directed to this bug with more current information (still getting used to Bugzilla) - I'm going to add 26211 as a see also https://bugs.koha-community.org/bugzilla3/show_bug.cgi?id=26211
*** This bug has been marked as a duplicate of bug 26211 ***