I agree with BigDaddy'sGirl - it's the rigor problem. I let mine "rest" for a minimum of 24 hours before freezing, or 48 before cooking if it's a fresh kill.
Also, meat birds, typically cornish/rock crosses, are butchered very early for really tender meat; 8-10 weeks; if you read 8 months, it was probably a typo somewhere. Raising birds to 8 months is not very good tender-wise, nor very economical!
One thing though, I have cooked some older birds in a water bath (sous-vide) and you can almost always guarantee the end result will be tasty and tender. If you don't have a sous-vide or water bath, cook very slowly in the oven, 180 f, yup, that low. Use a covered dish; tight fitting lid, and some liquid in the bottom. It will take several hours, but you can get reasonable results.
Most folk don't know about sous-vide; you can Google it. I use it for MOST of my cooking now, especially home grown ducks, geese and chickens. And it really is that good!