Age breaking pets...

At what age do pets need age breaking and how exactly does it all work?

Know I'm late with this whole thing and age breaking machines been around ages but I'm kind of a casual player and never got as far as trying it out.

I always thought the cut off was 100 and after that pets just wouldn't gain levels unless you put them in the machine but just had one go from 100 to 101 that I know for a fact hasn't been in the machine??

Have I misunderstood the point of the machine?

Someone explain like I'm 5 please? 😭