Yes, this is a good sign! I have just one of 7 pullets laying. She's a Barred Rock and the most mature of my flock. The rooster started to mate with her I'd say 2 weeks before she started laying. Different breeds start laying at different times and then of course, individuals each vary within a breed. Also, when their faces, combs and wattles turn a deeper shade of red, this is also a signal. Also, if you go to pet them and they "squat" this is a sign as well. Another sign is when they start taking more of an interest in the nest boxes. I assume you have them up and have put some sort of fake eggs in them (golf balls, ceremic, wooden or plastic Easter eggs)?
There are several really good and interesting threads here on BYC about these signs, search for "signs pullets start laying" or something like that. If you want info on nest boxes to make sure yours are set up as best they can be, search on "pullet using nest box" or something like that.
I did this research and based on advice from experienced people, I first made sure my nest boxes were open at 16 weeks based on the advice of experienced flocksters. I also modified my nest boxes a bit by moving them closer to the ground (I'll raise them back up higher once they get used to them) and made them darker by turning them to face the darker side of the coop. I already had the fake eggs in them and had them filled with straw. But I believe these modifications helped. My first pullet to lay started at exactly 21 weeks and she's laid in the nest box every day since. I free range and they spend a lot of time in the woods so I'm super happy she's using the nest box and again, I think these modifications were what did the trick.
My nest boxes contain straw, and I out some golf balls in them. I've heard that with golf balls, when they peck them it hurts, resulting in less pecking of eggs, when they come around. I have twelve neat boxes, and each one of them had been stirred up and sat in! Hopefully soon! Thanks for all the good info!