Male Cockatiel Aggression

We have 3 male cockatiels (no females). The youngest who is about 6 years old has always loved our 10 year old cockatiel and follows him around and gets very upset if he goes out of sight. All of a sudden the youngest has turned very aggressive toward the 10 year old to the point where we have to lock up the young one and the 10 year old tries to hide. What's going on?

It's January in Michigan - not prime time for mating behavior although I do know cockatiels mate all year around - we've had many babies when we had females in the house too. The young one has been regurgitating quite a lot on a small rug we have. Does this sound like a mating issue and if so do you have any suggestions?