At 18 weeks, they are getting close to laying eggs. So the combs on pullets start to grow quickly, up to the size and color they will be as mature hens.
In this case, I am pretty sure yours are all females, so no need to worry about returning.
In general, if you bought from a hatchery or a store, you cannot return a rooster. Some places will refund part or all of the money you paid to get him in the first place (but not the money you spent on feed to raise him), and some will not even do that. It's your problem what to do with the rooster (keep him, butcher him, sell him, give him away, have a vet euthanize him, etc.)
If you bought from a farm or an individual person, it will depend on what they say. Some of them do allow returns in case of sexing errors, but many do not.
But again, I think yours are all females, so your decision will be easy: just keep her and see whan the eggs start coming!