I've got 4 BOs, hatched early April....One started laying at the end of Sept (25wks), another at 28wks, third at 29wks and still waiting on the last. Orpingtons mature later but they are so fluffy & love the breed/personality.
We all know the combs/wattles get RED, then there's the squat but now I've learned the final sign is the pelvic bone spread. >2 finger = close & the spread gets larger 3+ fingers once they get started. They'll also show more interest in the nest box.