A couple of chicken questions I've been asked lately:
1. Do chickens get more or less healthy as they age?
Though they develop a resistance to some diseases, they also become more susceptible to other diseases that commonly affect older birds. Some factors that affect the health of your flock are bringing in new birds frequently, having neighboring birds nearby, and fitting and showing your chickens.
2. How do you tell whether your chickens are healthy? Check how they look: Are their eyes bright and lively? Are their combs bright, waxy in appearance? Check their feathers; have they retained their sheen? Check their legs and vents, as well as underneath their wings, no parasites? Great!
Unless it's unusually hot, your chickens should be at their normal activity level, moving most of the time throughout the day scratching around, preening themselves, taking dust baths, and generally pecking at whatever is nearby.
Listen to the sounds your chicks make, and when things are a little off-note, you'll recognize it. Normally chickens cluck and make contented sounds when they're healthy. Sneezing and coughing are signs that something's amiss. You'll get used to what's normal concerning the number of eggs each of your hens lays, if she suddenly stops laying or the eggs are thin-shelled or shaped differently than normal, that's a signal that she may be ill. Pick up each of your birds at least weekly, you'll be able to tell whether they're gaining or losing weight. If that's an issue, and it's not molting season, or the hen isn't broody, keep a close eye on her. Check their droppings, for unusual color or odor, as that's a common sign indicating illness. If a hen begins molting, egg production will decrease, so that's normal.