My chickens are 7 months old and I still haven't seen an egg. Why?
First, you should make sure they are on a good chicken feed. Once they have begun laying, switch to layer pellet or crumble; until then, feed them starter or grower. Some people make the mistake of feeding "scratch" only, when scratch is just meant to be a treat. Scratch in particular is mostly corn, which has very little nutritional value to it, and is low in protein. Feeding them scratch only would be like feeding your kids potato chips and corn chips only: it would simply not be very healthy!
Second, some breeds do not come into lay until later than others. Leghorns often come into lay at five or five and a half months; Egyptian Fayoumis can come into lay as early as four! However, others lay later: Easter Eggers sometimes don't begin laying until seven or eight months, and large breeds like Brahmas and Jersey Giants often take a very long time to mature. Six months is just an average time to maturity for hens. (An extra week or month will seem like forever, I assure you!)
Something else to consider is that hens do lay fewer eggs in the winter season, so if they come "of age" during this season, sometimes it happens that they don't begin laying in earnest until the spring. Some people will choose to use supplemental light for healthy hens in the winter time to increase winter production, however, most hens need to use those calories to help them keep warm during the winter rather than to lay eggs, so extra light is not always the best idea.
Another thing that might be occurring is that your chickens could have been exposed to parasites like worms, mites or lice. Wild birds could have brought something into your coop area or they could have been exposed to something when they were free ranging. Check carefully for parasites, as the anemia caused by a severe infestation causes production to go away (or to delay its start).
If they free range, another possibility is they might have begun laying, but could be hiding their eggs in a secret nest somewhere. If you think this is the case but you can't find the hidden nest, you can leave them in the coop for a few days to see if any eggs appear inside. On the other hand, if your hens don't regularly free range but are closely confined and bored, they might be eating their own eggs... or possibly a predator of some kind is stealing your eggs.
As you can see, there are a few things that could be going on, and you will have to observe them closely to be sure. If their diets are balanced and they are healthy, however, chances are good that you will see eggs between 4 and 9 months, depending on the breed and the time of year.