If I add an age to PatronSelfRegistrationAgeRestriction and save, but then want to remove the age to disable the system preference, 'null' won't save properly. I get a 'Nothing to save' message, and the original value is still enforced. If I then leave the page and later return to the system preference, the entry box appears null even though the original value is still being enforced. As a temporary (and unintuitive) workaround, I can save 0 and still self-register patrons of any age.
Bug 40405 seeks to move away from NULL system preferences, so I suppose we really want to blank this instead? I can confirm that setting the preference to 0 makes it act as if unset, rather than requiring all self-reg patrons to be less than 1 year old. This is convenient and makes sort sort of sense, but it's clear enough that I'm happy to rely upon it as a solution here.