Raising chicks from babies to healthy chickens is not the easiest thing to do in the world, but if you are successful it is very gratifying and will give you some amazing pets. It only takes a month or so to see some significant changes in your birds. Between age 3 and 6 weeks your chickens will shed their fluffy feathers and grow new feathers that are more mature and better suited for the dealing with the weather. Their combs and wattles will also grow during this period and become red.
Soon your cockerels, young roosters, will start to attempt to crow. At around 21 weeks old your pullets, the young hens, will start to lay their first eggs. Pullets' eggs generally have very weak shells and are small. As your young hens start to lay more eggs they will get larger and have thicker more durable shells. After six months your young chickens will start having fun with each other and start pecking one another. By this time they will have fully formed wattles and combs.
After six months though, they will start to slow down and their production of eggs will slightly decline. Molting will continue once a year and there will be no egg production during this period. Molting is the process of shedding their feathers and re-growing them. This usually happens during the summer and your chickens may look ill because of the loss of feathers, but don't worry because this is normal. If it takes some chickens much longer than others to grow back their feathers, you may want to bring them in to get checked out.
Another annoying behavior that you may have to watch out for when raising chickens is called broodiness. This is when your hens get stubborn and decide they are going to sit on their eggs all day long. This is fine if you want the eggs to hatch real fast, but once they get into this mood they will actually sit on anything that resembles an egg. When they are broody they will get angry quickly if you try to bother them or get their eggs and will likely peck you. Your unfertilized eggs that you want to eat or sell will decompose much faster if the hen stays on them. Your hens will also likely not leave their nest even if they are hungry or thirsty and this could lead to them becoming malnourished and getting sick.
I hope that you have as much luck as I have with raising happy and healthy chickens. Make sure you have built a proper chicken coop with the necessary chicken run that gives them enough space to roam and get exercise. Once your chickens are fully grown they do not require too much attention to keep healthy. Just make sure they are properly fed and their water supply stays full. The last thing you need to do is make sure you clean out the chicken coop regularly so they do not get sick from contaminated air. Good luck and have fun!