My dark brahma is 25 weeks and she has not laid an egg yet. She just is starting to become very noisy, so I imagine it will happen in 3-4 weeks from now, perhaps sooner.
Unless you bought your Americana from a breeder, you have an Easter Egger, which also take longer to lay. My eldest Easter Egger is 25 weeks old and she laid her second beautiful blue/green egg this morning. Her first egg came two days ago.
Well worth the wait! My RIR lays the most with 6 eggs a week, which she started to lay at 23 weeks of age.