My youngest hen (the gold/cream welsummer/ee cross) laid her first egg at 23 weeks old. I am still waiting on my SS Hamburg hens though and they are almost 24 weeks.
Her egg is on the right, her mothers is on the left
I have 2 Aust pullets that are 28 weeks old and have not laid yet. My SL Wyandotte finally laid her first last week (30 wks old). Our RIR's laid at 29. The older hens we have were given to us after they started to lay, so I don't really know how old they were then. I think it was 22-24 weeks though.
Edited: had to change teh ages because I forgot the SLW and RIRs were 3 weeks older than the Black Aust's.