Okay, nevermind the original question..
I decided to take a look at the list of EE addons, and there’s a really old extension that still works and does exactly what I needed it to do - add the birthday field to the registration page and make it mandatory.
I’ve also removed the ability to change the birthday from the member profile page by removing the table cell from the Member Profile Form template. It seems the admin still has access to changing it which is good.
The last thing is that, since this is normally an optional field, I want to give users a choice as to whether they want to display it publicly or not. Going to the public profile, it is displayed by default, with no options to turn it off. I don’t want to completely remove that field though, in case they DO want to display it. In the Public Member Profile template, right above the birthday field is an IF conditional for displaying forum statistics. This seems like a good idea if I can manage it for the birthday field. Could I create a custom member profile field for the user to select yes/no and use the result of that as the conditional to display/hide the birthday field?
Thanks
Edit:
Dangit, I keep running into problems.
I successfully made a custom profile field to run a conditional IF against to see whether or not to display the birthday in the public profile - this works.
However, I overlooked the fact that when the user goes to change this option in the profile, it would try to look up whether they changed their birthday as well, which results in it not finding anything since I removed that field from the profile page, and thus updates the database with 0’s.. I’m trying to find a way to work around this now - to prevent the birthday from changing.