As far as I know, hybrid chickens can be expected to live for 3-4 years. They lay for most of this and don't live for too long after they finish laying. Pure bred chickens can live for up to about 10 years, don't lay so many eggs and only lay productively for 3-4 years, then slow down, but can keep laying for up to around 8 years in total. Hybrids are generally more friendly, but some breeds, such as Cochin and Orpington are very friendly and docile, although very big and don't lay many eggs.
Most people buy point of lay (POL) pullets. (pullet is the word for young hens that haven't started laying yet). Some might be just laying or they should start laying in 2-6 weeks. Hybrids start laying sooner than purebreeds in general and most hens lay their first egg between 18 and 22 weeks, but it can be later. They aren't scary, they will run away at first if you try to touch them, but you can train them easily to come over for food when they see you. They'll eat from your hand etc. Just before they start to lay they may 'crouch' when you approach or put your hand on them, which makes them easier to pick up.
They are great pets for children. Mine love checking for eggs. We are also getting lots of double yolkers at the moment as three of our pullets have just started laying, so the children think that's fantastic and we had a treble yolker the other day!
You don't have to clip their wings, but they can get over fairly high fences if you don't. I have clipped one wing of all of mine except the Cochin (Cochin are too big and heavy so don't fly) as one of our fences is only just over 3ft. I suspect they could still get over the fence if they wanted, but they're happy to stay as they have everything they need with us.
Cats will be interested in the hens at first, and will take young birds, but POL pullets and adult hens should be fine. Our neighbours cats 'stalked' the hens to start with until they had been chased off a few times, now they know to keep out of the way.