I think my hens are older than yours, the guy said around 3 years old, but could be older. they belonged to an older guy who gave up his allotment so the lad I got them from took the birds in first and had them a while. so I don't know what to think.
There is a place further down south from me that sell rescue chickens from the battery before cull, they are 72 weeks old, and I was thinking about getting some of them, but when these came up I jumped at the chance to own my own chickens.
I just don't know what to do really, if they stop laying then they have served their purpose to me as I have them like you do for the eggs. I can't afford to feed 8 hens and get nothing back from them so culling is something that would have to happen.
18 to 30 eggs a day is an amazing amount of eggs, I don't know what I would do with that many, I have had to start to give them to the family as my fridge was starting to look full, and there is only so many eggs I can eat lol.

I do a lot of DIY, I've made planters for my plants that only cost the screws, the wood was from pallets, and these planters were 8 feet long by 2 feet wide and 3 feet tall, it took some doing, then I made one just smaller than that to go next to it, this was what I was using for a fence to stop the scrap man from thinking my garden was his to walk over.
I've made fairy houses for the kids and built shelves from pallets, I just like to use my hands.
I should never have gotten the chickens first but I did, but before that, I should have built the run and coop then it would have looked a lot better than it did, and with having little money I had to build it cheap, so I decided to use recycled materials.
It's nice to chat with another chicken keeper, there are some neighbors of mine that like to tell me what to do with rearing them, and they can't properly look after their own live stock, so having a good natter on here makes me so much happier. hope your keeping well :D