Ours are not pets so we re-home them when laying slows down, generally around 2 years of age. I really notice the slowdown during their second winter, generally speaking.
The way you can tell if a hen has layed an egg recently is to check the pelvic bone spread. Pick up the chicken. Turn the chicken upside down by putting your fingers between its ankles, hold on tight, and turn over...holding on to the ankles. Then cradle the upside down chicken in your arm. Locate the vent. Then using your index and middle fingers spread flat, move your finger tips from the vent towards the tummy. Within an inch or so you should feel a pointy bone on each side of your fingers. If you can fit at least two fingers between the bones the bird has layed an egg within the last week or two (about?).
Mine don't seem to notice when some go away. They just rearrange the pecking and roosting order.
A bit too small but depending. If you have generally temperate weather and they are outside most of the time it's fine. Since I just put our new pullets in with the existing flock we have about 28 birds in an 8 x 10 shed. They do just fine. Just be sure you have enough room for everyone to get to the feeder and waterer so that the low ranking birds get their fair share. Also be sure you have enough roosting space for all of them.