I think it has to be a customized calculation.
I start with the value of the hen based on breed, color, productivity, egg prices, and location influences.
Then average the useful laying lifespan of the breed.
2 years for layer hybrids, up to 5 years for some heritage, and somewhere in between for others.
Calculate that into a percentage reduction for each lost year of laying life.
So say my homebred black EE's are worth $30 at POL (previous years). And they lay well for 4 years. So I'll take off 20-25% for each year of age, though I've never tried to sell older than 3 years.
I make a minimum of 3 because they will have bonded with friends and it's hard to change families with a quarter of your life gone by. If I have 5 to sell, they all have to go together since I wouldn't be able to make another group of 3.
That's my formula anyway.
Now, some people with a traditional farmer mindset might see older hens as without worth.
But I have found pretty receptive folks in my area to 2 year old hens with longer laying expectations... Most of them fairly new to chickens. Because it can seem daunting to raise from chicks, or they may have had bad experiences with chicks and losses before. Or the old "they were supposed to be pullets" headache.
A mature hen is not only going back into lay after she settles in (1-3 weeks in season) giving quick egg results, but she lays larger eggs of a known color. She also has a proven, developed immune system. And may be more likely to go broody.
That's one of the reasons I keep some older hens, they often make better broodies.