Based on how you would cook chicken, can you recommend an age that's best for different cooking (baking, grilling, roasting, etc)?
That is a fair question but no, I don't have ages for cooking methods. There are too many variables. Some of that is your personal preferences and what you are used to. If you are used to the store chicken, that's the Cornish X butchered around 6 to 8 weeks of age. You might find a 12week old cockerel fried kind of chewy where someone else would be OK with a 14 or even a 16 week old. You might be OK with an older fried bird but your kids or spouse may not be. The same thing is true of other cooking methods.
How you manage and feed them can make a difference. Part of that is diet, part is how much exercise they get. A couch potato will be more tender than an athlete.
As they go through puberty the hormones cause the boys to increase in texture and flavor. Not all cockerels go through puberty on the same schedule, some start a lot earlier than others. Some of us like that flavor, some call it gamey and hate it. An early maturing 18 week old can be a lot tougher than a late starter.
Pullets don't go through that hormone thing like the boys do. The girls will gain texture as they age, an old hen can be really stringy if not cooked right. But with the girls it's a really slow process, with the boys it can be really fast. Do you eat the girls?
Aging, brining, and marinades make a difference. All birds should be aged until rigor mortis passes. If you don't age them, any of them can be really tough. There is some debate on here as to whether brining tenderizes them or just causes them to retain moisture. Retaining moisture can help if you use a dry-cooking method but doesn't really make a difference if yo use a wet method. The acid in a marinade breaks down fiber so marinading can make a difference in texture. How strong the acid is and how long the meat is in the acid controls how much the fiber is broken down.
Details of your cooking method can make a difference. I suspect frying in a deep layer of lard might give different results than trying a thin layer of other oils. How much do you baste when roasting? When I bake I use 250 F, I don't go as hot as 325 F. What temperatures do you use?
Another reason I hesitate to give ages is that I don't use many different methods. I don't fry or grill my own chicken so I don't have that experience. I'd think you'd be OK frying or grilling to 12 weeks, 14 weeks might work. I bake my cockerels up to 6 to 7 months and my pullets up to 1 year. After that I tend to go to other methods.
My general suggestion is to use trial and error. Try something. If it doesn't work try something else until you find your sweet spot. When you start out try to be cautious but as you gain experience get more daring.
Good luck!