First, when you butcher be sure to age them until rigor mortis has passed. If you don't they will almost certainly be inedible. That can be true for any chicken but especially for older roosters.
As they age the roosters' meat gains flavor and texture. The flavor can be described as gamey, same as any other older intact male like bull or boar. Some of us like that flavor in chickens, some don't. Cooking it with other strong flavors can help with that. With one exception, you need to cook the meat slow and moist. That exception is pressure cooking, that is not slow. Give up on the idea of grilling, frying, anything like that. Not going to happen.
One way the French turn an old rooster into t gourmet meal is Coq au Vin. Literally Cock with Wine. Marinading in wine is an essential part of this recipe. The acid in wine breaks down the tissue that makes it tough.
Molpet mentioned pressure cooking, enough said.
Putting the meat in a crock pot and cooking it on low overnight can give you some really tender meat. Flavor comes from what you add.
I like to make broth with them. Put the meat with water, an onion, garlic, carrot, celery, and whatever herbs and spices you want (I grow basil and oregano so I use those a lot) and a dozen peppercorns in a crock pot. Maybe a bay leaf. I cook that on low for about 24 hours. Pick and shred the meat for tacos, soup, chicken salad; I make sandwiches for lunch. Strain and defat the liquid for the best broth you've ever had,