I agree that probably most people would say they are adult before they reach 4 yrs old. Most people will breed a female that is about 2 1/2 yrs old if she is big enough, and some people will breed them younger than that. Males mature faster.

Yearling specifically refers to an animal that has past its first birthday but not its second birthday when you are talking horses and I believe other livestock. I'm not sure BP people use it the same way, but since the meaning is pretty obvious from the word, I would think so.

I've also heard sub-adult quite frequently.

Also, don't forget that different people use the terms slightly differently. What one person means when they say juvenile might be exactly the same as what someone else means when they say sub-adult, while yet another person might think those 2 terms mean something totally different.

The fact that male mature faster further clouds the issue, because a 2 yr old female might be a sub-adult, while a 2 yr old male might be an adult proven breeder.

So, you've got it pretty close, but you can't be sure unless you ask each time.