Chickens need to eat grit to help digest their food. The grit helps grind down the food the hens eat because as everyone knows, chickens don't have teeth (hence the term "as rare as hens teeth"!) If your hens free-range then it's likely that they already get enough grit in their diet, though if they are contained you should provide some grit (it's often sold in pet shops as grit for pet birds or pigeons).
Hens also need enough calcium in their diet - they use calcium to form the shells on their eggs so if they calcium deficient you will get soft eggs, rubbery eggs or even eggs with no shell. You should provide oyster grit in their pen or put some in a small food bowl. This will help them lay hard-shelled eggs and is an essential part of their diet. Some all in one feeds such as certain layers-pellets will already contain a supplement of oyster grit, so it's worth checking.
You should try and make sure your hens have fresh food and water available at all times, and they often like to go to bed with a full crop! You can leave their food out overnight but remember it may attract vermin such as mice, rats and slugs, so depending on your circumstances you might have to act accordingly.
With the right care and attention, chickens can live to be fifteen years old, although the average life span is five to eight years old, depending on their weight and breed. They will lay eggs well for the first two years and then for the rest of their lives you will probably get an egg every so often, but they'll continue to make great pets!