Some Australorps are early layers, some start later. If you had enough for averages to mean much, you'd see some lay early and some start later. But you only have four. That's not enough for averages to mean anything.
I've had individual Australorps start laying before 20 weeks. I've had individual Australorps start well over 6 months. Each one is an individual and will start when they start. Breeds have tendencies, production breeds tend to start laying earlier than the decorative breeds. That does not mean every production type pullet will start laying earlier than every decorative type pullet. Some decorative pullets can start pretty early. People tend to get hung up on breeds when they are just chickens, each with their own individual personality and traits.