They are not going to be laying at 12 weeks. Some might be laying at 18 weeks, some could take a few months longer to start. It can vary a lot with individual chickens, but I’d expect most of your Australorps to start laying somewhere around 20 to 24 weeks old. I’ve had some start at 18 weeks, some at 30 weeks.
We don’t all do anything the same on here. That’s part of what makes it confusing. There is not just one way that works and you have to do it that way. There are a lot of different ways that work and we do all of them. You have to decide which way best suits you. I’ll eliminate some choices. You are beyond some of them since you have already switched to Grower.
The main thing you have to worry about with feeding growing chicks is that too much calcium can harm their internal organs. Layer feed has extra calcium so the hens that are laying eggs can use that for their egg shells. So you should not feed Layer to growing chicks.
One standard method is something like you described. Feed them Starter from hatch until they are about 4 to 8 weeks old, then switch to Grower. Then you feed Grower until you find your first egg. That’s when you switch to Layer. Some people don’t wait for the first egg. They automatically switch at 18 to 20 weeks whenever that bag of Grower runs out. Some should start laying soon.
Other people will feed Grower forever and never switch to Layer. To provide enough calcium for the egg shells when the hens are laying, they offer oyster shell on the side. The hens that are laying eggs seem to know instinctively to eat enough oyster shell for the calcium, but the ones that do not need the extra calcium may take an occasional bite but don’t eat enough to harm themselves. This is a standard way to feed a flock that has mixed ages, some hens laying and some growing chicks.
Instead of Grower, some people might feed them another feed. That might be a Flock Raiser, a combined Starter-Grower, or something else. There are a lot of different products out there. The main difference in these is percent protein. The percent protein might be anywhere from 16% to 20%. It really doesn’t matter which. The only rule is that you do not fed Layer to growing chicks.
There are other things you can give them to supplement the calcium if they need it. Oyster shells are the typical calcium supplement because they are plentiful, pretty inexpensive, and they work really well.
There is something else to consider about the oyster shells. If all they eat is Layer, the Layer has all the calcium they need for their egg shells. But if yours forage a lot or you feed them a lot of low-calcium treats they may not be getting enough calcium for their eggs just from Layer. It is often a good idea to offer them a separate container of oyster shell in case they need it. If they are getting enough calcium from other sources that oyster shell may last for years. If they need it, then it will gradually disappear.
So the short answer to you is to switch to Layer when you see the first egg but offer a separate container of oyster shell on the side. That will work fine. But do not start to worry if you see that someone else dos it differently. There are a lot of different ways to do this.