My hatchery Australorp started laying late. At least 24 weeks...maybe later.
I also heard that hatchery Australorps have lighter colored eggs if they have Leghorn mixed in, but mine has dark eggs...even darker then my BSL and RSL. So maybe she doesn't have any Leghorn in her?
I found out that it WAS my Brown sex link and now my Black sex link started at 23 weeks. I'm still waiting for my Australorp. She's 24 weeks and her comb and wattles are still really small and pale.