[Casey76]

Well, just for comparison... this is a photo of my LS at 8 weeks



and at 19 weeks



Unfortunately I was never lucky enough to get any sussex girls, so I can't do a comparison there.

One thing though, I will confirm that your bird isn't pure breed   A LS sould have white legs, not yellow.

Given that sussex tend to have large combs and wattles, compared to orpington, I'd still hold out that your bird is a pullet.  But it would depend on what it was crossed with as it does have quite a cockerel-type stance.