I have both of those breeds, and I think they took a bit longer than 20 weeks. Some signs you'll notice when they're getting ready to lay are fussing around the nesting boxes, redder combs, and this squatting that they do when you reach out to pet them. I've found that once they start squatting (it's a mating pose), then eggs will be coming real soon.
Around 17 week, I would put some nice straw in the nesting boxes for them, and even add a golf ball or two.
I have 1- Australorp, 1- RIR, that started laying about 10 days ago....eggs are very small. the other Australorp, 2- Wyndottes, 1 - Brahama and 1- EE have not begun to lay yet. They all are 22 wks old.