Age and activity level absolutely effect how "tough" meat is. I've raised and cooked both heritage breed chickens and modern broilers. The former take 2-3 times as long to reach maturity, are far more active, and as a result are "tougher". You can cook them just as fast a broiler, but the end result is going to be tough/chewy meat. The longer cooking time is needed to achieve equally tender meat.

It's the difference between cooking a rib-eye and a brisket.