The typical pattern is for them to lay well for about 1.5 years, then go through a molt, then start laying again. When they start again, they usually don't lay as many eggs, but usually they are larger eggs. They may lay for several years, continuing to slow down as they get older. I doubt your girls are through laying, but they are probably through their highest productivity. I have some that are over 3 years and all mine lay, though not every day. I really don't keep track, though, so I can't tell you accurately how many each one lays. I have 10 hens between 1 and 3+ years old, and probably get an average of 5 eggs a day.
If there is an average age when they stop, I don't know what it is. I've read of hens being 8 or 9 years old and still laying a few a year.
