The person I got mine from said they were 5-months old, and they started laying one month later. If I got correct information that would make them 6-months at laying time.
I have heard that 'hatchery' barred rocks lay more frequently and earlier than full bred barred rock chickens. Perhaps mine are part mutt, they don't have bright yellow legs and beaks like the breed standard.
If your hens start a little later, it may pay you on the back end and they may keep producing for a longer laying span than the early starters. As frustrating as it is to wait, it may end up being worthwhile. Perhaps the person that you got them from could clarify if you still have availability to talk to them.
good luck with the outcome.