Looks like a pullet to me. Some just develop a little faster than others. I don't see any saddle feathers and the comb looks pale. If she were a he you would definitely be hearing some crowing at this point and (he) would be mating the pullets.
My RIR's are 3 months old and I say yours is a hen. Our 2 Roo's are very obvious at this point. They are crowing have green tail feathers and big beautiful combs.
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Glad to hear I was wrong. Since my latest EE turned out to be a roo, I'm seeing them everywhere. But I agree, by 18 weeks if it were a he, he'd be crowing and doing that weird wing stretch and fluffing up his neck.