As far as the best age to butcher roosters, another factor is how fast their testosterone is coming in -- i.e., how fast they are maturing. Some cockerels begin crowing and chasing hens at 11 weeks or so. Some don't exhibit this behaviors until the are 16, or even older.
In my experience, a 15 week old cockerel who has been crowing and attempting to mate for a couple of weeks, will have a firmer texture and a stronger taste that a 15 old who has not exhibited those traits yet. We have also observed that the testes of those more mature acting cockerels are also much larger then the those of the more mildly behaved, and more tender tasting, cockerels.
So, it's always a dance between size and tenderness. If you want a cockerel that is tender enough to grill, I would advise butchering either when they first start to show an interest in crowing and mounting hens, or by 14 weeks, whichever is earliest. Raising cockerel late in the season, so that they are coming into maturity during the late fall and winter when daylight hours are short, can also be helpful in delaying testosterone development until they are larger in size.
As far as the hens go. This year, I started butchering pullets for the first time, as part of plan to have a true dual purpose flock. Like
@Ridgerunner I don't have the space to do that unless I'm willing to eat extra pullets. I am breeding for size, and intermingling some Red Ranger genetics, so my pullets are bigger than average. With that caveat, I've found that a 6 month old pullet is plenty big to be worth the effort and still quite tender -- much more so then a 6 month old cockerel would be. I thought that the flavor too, of the pullets was excellent.
Part of the process is experimenting to see what works for you. You might also need to relearn your expectations of what a chicken "should" look or taste like. No dual purpose bird is ever going to look or taste like a CX butchered at 6 weeks old. The further I've gotten into this, the more I've really come to enjoy and appreciate the extra texture and taste of the birds I'm raising.