What age do your hens slow or stop laying?
Each chicken is an individual so they are not always identical. Breed has something to do with it and so does breeding. Some breeders pay attention to longevity of laying so they can affect how long they lay well.
Studies have been performed on the commercial laying hybrids. Somebody has to pay for these studies, I have not seen any studies on the standard dual purpose breeds or decorative breeds that we keep. With the commercial hybrids, a typical pattern is that they lay extremely well their first laying season. After their first molt they lay really well until their second adult molt.
After their second adult molt production drops maybe 15% to 20% on average. You have to have enough for averages to mean something. Any one individual hen may continue to lay really well while another may lay a lot worse, but the overall flock average is fairly consistent. That basically means that is a hen is laying 6 eggs a week on average, she only lays 5 a week after the second molt. With the tight margins the commercial operators work on that is significant.
Yours are not the commercial hybrids so I have no studies on them. How many molts have your chickens been through? At 2 years old I'd think one molt, maybe two? How long has this drop in production been?
There are different reasons a flock may drop in production. With yours at two years old I seriously doubt it is age related.
It is October. If you are in the Northern Hemisphere and unless you are manipulating the lights so your chickens don't know the days are getting shorter the vast majority of 2-year-old hens should be molting. This is why I ask about the laying history, how long has this drop been going on? If you are in the Southern Hemisphere then this is different.
Another possible reason is that they are hiding a nest or nests on you. That happens a lot.
Another possibility is that something is getting the eggs. I don't know what continent you are on so I don't know what your potential predators are. Many critters that eat the eggs leave signs behind, broken shells or wet spots. Some critters don't.
Certain diseases or parasites can cause a drop in egg production. It is always a good idea to check for mites, lice, or worms. But if they are acting healthy they probably are healthy.
Sometimes stress can cause a drop in egg laying. That might be from extreme heat or cold, a predator attack, a change in the pecking order, running out of water for a few days, or a change in housing or the daily routine. Usually these don't last that long but sometimes they can throw some into a minimolt where they stop laying for a while.
So what can you do? If yours free range you can try locking them in the coop or coop and run for a few days, provided you have enough room. If production goes up you have kept them from hidden nests or have locked out an egg eating predator. Or try marking a couple of eggs and leave them down there. If they disappear you know something is getting the eggs.
I don't know what is going on with your flock, but since
most of which are a couple years old now
I really do not think it is age related.
Good luck!