Two years is usually the age because that's when OFA's are official. On top of that:
A good bitch can easily be finished in the AKC ring by 2.
A person with the intent to breed and Schutzhund training skills, can easily have a SchH1 by 2 (I know someone that had a SchH2 by 2).
Many females don't ever get to SchH3 anyways, so people just get a 1 and breed away.
It has less to do with maturity, and more to do with having a dog that's ready on paper.
I also wanted to add...if you do it wrong, there's a lot of money in breeding. If you're just breeding your one dog, you definitely make money. Even if you consider the costs of trialing/training, you're making money. Like if that's your hobby, and you love to do it, and you train in SchH because you love it and not to "prove your dog's breeding worth," you're going to spend that money anyways, so if you breed your dog, and end up with a nice sized litter, you'll recoup that money and still be ontop (assuming the money was going to be spent anyways). Now...if you don't like SchH, and you're just doing it because the SV says you have to in order to breed your dog, yes, in all technicality there isn't any money in it.
My suggestion would be to join a club...any club. It could be a GSDCA club, SV, Schutzhund. And learn from people. See what they're doing. See what they're trying to accomplish. Understand why they want to breed this dog to that dog. You'll quickly pick up on the basics, its the more advanced things that you really need to start learning. After 3 years at my local GSDCA club, its amazing how much I know and how much better formed my opinions are.