My hens are five and lay maybe one egg a week each — what do people actually do at this point?

This is just what the curve looks like. Mine peak in their first full laying year, drop noticeably in year three, and by five they are ornamental with occasional bonuses. The birds themselves usually have years left in them, I have had hens reach eight and one hybrid make ten, though hybrids bred hard for output tend to have shorter, rougher retirements than heritage breeds. I run a rolling flock: two new pullets every second year, so I always have someone laying and nobody has to make a decision on a specific bird.

The answer nobody enjoys is that most people who keep flocks long term do dispatch birds at some point, and they are usually quiet about it because of threads like this one. If you will not, that is a legitimate choice, but then your six bird limit is really a two bird limit for new layers, and you should plan around that rather than being surprised in two years. Decide the cap now, in writing, or the flock decides for you.

Since nobody has said the number out loud: at four eggs a week from four hens on roughly 15kg of feed a month, you are somewhere around a couple of pounds per egg once you count bedding. That is fine, and I say that as someone who keeps two retired birds on exactly those terms. I just think it is better to know you are paying for pets than to keep telling yourself it is a food system, because the second story makes you resent the birds.

Five is not old for a hen, it is old for a laying hen. Feed them a lower calcium maintenance ration if they have essentially stopped, watch for the ones who go egg bound after a long gap, and enjoy the fact that old hens are much better company than pullets.

If you do add pullets to five year old hens, the old girls are usually less aggressive than a flock of two year olds but slower to accept anyone. Mine took about four weeks with a see but do not touch panel, two feeders and two waterers at opposite ends of the run, and I did not let them mix unsupervised until the pullets could outrun them. Do it in spring so the pullets come into lay while the weather is on your side.