If they're still laying well, no need to send them to the stewpot yet, especially considering how long you'll take to get replacements in the coop up to speed on production.
Here's a good article on cooking heritage birds. One of the points made is that for older birds, cooking longer at lower temperature is better to keep the meat tender.
With commercial poultry we're used to cooking everything for an hour at 350, this recommends 300 - 325 and a half hour per lb.
Here's what it says about older birds, also known back in the older days as "Fowl."
Edited by DenverBird - 9/6/11 at 12:56pm
Hens and roosters butchered at older than one year, classified as fowl, make very fine eating also. This class was perhaps the most commonly eaten and least seasonal type until the mid-20th century. But today mature fowl is rarely available, unless you keep your own flock or know a farmer who does. It is essential to use moisture and low temperatures in cooking hens and roosters over 1 year old.
It will take hours longer to cook fowl, but the meat is richly flavored and was esteemed for sandwiches, chicken salad, pot pie and all recipes calling for cooked chicken meat. Fowl will become just as tender as younger chickens as long as it is kept moist and the meat temperature is kept low, preferably below 180 F. If the meat temperature goes above 180 F, the protein fibers toughen so that even if it is cooked long enough to fall apart, the individual fibers remain tough. When stewing, the water should not be allowed to boil, but should be kept at a simmer temperature, 180 F or less. Fowl can also be steam-baked with 1 or 2 cups water added to the pan; the pan should be tightly covered so the moisture won't escape, with the oven temperature at 300-325 F. Whether stewed or steam-baked, the breast meat of fowl will be best (especially good for sandwiches) if it is removed as soon as it is done, which may be a couple of hours before the dark meat is done. I allow at least 3 hours to cook a 3 1/2 to 4 lb hen.