Dew - when my chickens abruptly and mysteriously stopped laying, I blamed everything from possibly neighbors helping themselves, to rats, to pretty much everything else under the sun. Like you they were two years old, so didn't think they'd stop producing.
I discovered that even though I'd checked the pen etc. 1,000,000 times in search of eggs. (Seriously, its a run and coop. How can you hide an egg in that?) It turns out, my chickens are way better at hiding them than I am at finding them. I eventually discovered a HUGE nest with over 60 eggs in it. -.-
Bonus: One of my hens is apparently flight capable, and it turns out she was flying through the open side of the run, over into my neighbors yard, and making a deposit behind a piece of plywood leaned up by her house. The gardener found the stash and came over to ask me if they were mine. Gave her a finder's fee of a dozen (fresh, clean) eggs, and she went from annoyed to excited.
I doubt its your chickens, as mine are now 3 and still laying like champions.