I had the same concern - I was expecting my two leghorns to start laying kind of early but they were 23 weeks (both laid on the same day). Here they are getting ready for the big event:
My white leghorns started about 20-21 weeks, but I'm still waiting on my RIR that are the same age to get consistent. Got 3 white leghorn eggs yesterday & only 1 RIR egg. I have 3 white leghorn hens & 10 RIR hens. The RIR's need to start earning their keep.... lol