Generally, it's just a matter of the tag wranglers using their best judgment and users hopefully letting us know if something is off. Right now, behind the scenes, we have a way of saying "these two things mean exactly the same thing" but not "these two things are similar and should be grouped together somehow but are NOT exactly the same" - we are working on ideas for the latter, though! There are a lot of places where that would come in handy.

('Underage' is actually a special case because it's an official archive warning, but if you want to add more nuance, you can definitely use the freeform tags for that.)