I don't have Coronation Sussex but I have some experience with chickens. Some pullets will lay a lot earlier or later (sometimes by months) than others of the same breed, even if they were hatched together and raised together. Each pullet is an individual and will lay when she lays.
Breeds do have general tendencies, that can influence when she is more likely to start laying but it is absolutely not a guarantee. If you only have one pullet it doesn't mean a lot. You have to have enough individuals before averages have any meaning.
If you had 100 C. Sussex and 100 C, Orp pulelts, you might be able to say that your Sussex, on average, started to lay before your Orps. But that does not mean your first egg was from a Sussex or that your last to start laying was an Orp. Breed tendencies apply to averages, not individuals.
Another factor is heredity. If the person that selects which chickens become part of the breeding program uses early egg laying as a selection trait, they will wind up with a flock that starts laying earlier (on average) than a flock where that is not a selection trait. They are still the same breed but they can have different traits in some things.
I understand people want hard and fast answers. It can be hard waiting on those first eggs too, I once waited nine months. But one reason you get so many different answers is that we have such a wide variety of experiences. You don't always get the same results. Your chickens are unique. Hopefully it wont be that much longer for you.
