I've had more than my share of old hens that surprise me every other year or so with a string of eggs, as my ten-year old GLW Su-su did some weeks back, or in the case of my eleven-year old SLW Lilith, a persistent failed attempt.
Lilith, age eleven, tries every year to lay eggs. She's pretty agile and active for her advanced years, but her shell gland is SOL of calcium. Her eggs are shell-less and end up breaking inside her body. That sets up an ugly chain reaction of vent prolapse and infection. This recent episode took eighteen days to resolve. Her previous bout last year required nine days to resolve. I didn't think I'd be able to save her this time, but she surprised me again.
I understand why people cull old biddies when they quit laying, although my chickens mean more to me than just egg sources. Maintaining old hens is not for the feint of heart, though.
As I recently explained to another person who was concerned about their chicken being able to swallow a large pill, chickens are built to swallow large chunks of food whole. Even my half grown chick that is sick at present can swallow a large pill easily.
People have trouble swallowing large objects because we must chew our food into small bits as the first stage in our digestive process. It's why we have teeth. Chickens don't need teeth because their digestive process begins after they swallow their food. The crop breaks down some of the large chunks, but the real grinding takes place in the gizzard, the next stop after the food leaves the crop. It's why chickens don't need teeth and why swallowing a large pill, a whole mouse or a toad requires no effort.
Slip the pill into the beak and it will disappear like magic. You won't even notice the chicken swallowing.