Sorry for the delay in replying. A busy weekend!
Layers are very expensive to feed, it takes a hell of a lot of food to produce eggs. As they age they lay a little less frequently. Unless people are prepared to pay more for eggs, poultry farmers cannot afford to keep laying hens past about 15 months of age.
They could live between 3-5 years easily, and some older breeds can be 8-12 years old (old mixed use breeds are heavier and more robust so live longer)
Over the past 20 years, increasing numbers of people have adopted retired layers. The hens can live happy healthy lives and lay plenty of eggs for a family who are happy to regard keeping chickens as “pets with a side benefit.” However, counting the cost of feed and housing, it’s not a cheap way of having eggs.
(Those of us who keep hens don’t care, we love our resident dinosaurs!)
Adopted birds are only a proportion of layers reared and culled each year in the U.K. Most are not adopted.
Eggs are easy nutritious and healthy forms of protein but there is cruelty as part of egg production. Male chicks are all killed as day-olds. Hens much older than a year are killed too.
The alternative to this is for everyone to pay more per egg. Then it would be economical to rear far fewer layers and keep them for longer. The males would still be killed, though.
As with all issues of consuming animal products, people have to find a solution that fits their ethics. I’m ok with culling day olds and keeping the spoilt divas in my garden for fresh eggs, but I recognise not everyone has the space or resources to do that.
Sorry for the long answer