Hard to see the shape of feathers on a white bird in a washed out photo. Maybe it'd help people ID if you held some colored paper under those feathers to show the shape. Hehe. XD
Looks like a hen to me. 18 weeks is really old to not be showing any clear signs of being a rooster. I have never seen a rooster that wasn't at least trying to crow at 14 weeks, most of mine have started crowing by 10-12 weeks and all of them we were certain long before that as they were mating and fighting with other roos.
I can see the saddle feathers... but aren't there some breeds that the ladies get those too, just not as long as the roo? If that's the case then I'd lean toward girl too... but I really haven't a clue.
I see saddle feathers, comb/wattles look to me like cockerel. But, it still has a feminine appearance to it. I want to say cockerel. Is there any way you can pen him/her off separately in a larger pen with a hen and see if he tries to mount her after they settle into the cage? Otherwise, the only other way to tell for sure is to wait until it either crows or lays an egg. I hate when this happens, when you get a bird that has you undecided, but I will say that in every one of my undecided cases it has turned out to be a male for me.
Looking at the 3rd pic again. I definitely see saddle feathers and I believe I see a sickle feather growing out over that juvenile tail. I bet if you checked the tail you'd have a bunch of blood feathers coming in, looks like he's in transition from juvenile tail to adult tail.