There must be about a hundred threads with info about how to cook older, tougher birds, and many, many different ways to use the meat.
Cornish X's will be tender up to around 12 weeks or so, they're usually slaughtered at 6-10 weeks. So it's no surprise that those are not tough, even without special treatment.
Others, heritage, dual purpose, mixed breeds of whatever kind, need slow, moist cooking. They have superior flavor, but you can't just throw them on the grill or in the skillet like a broiler.
8 months is not considered young for a meat bird.
6 weeks is what Cornish X are bred for, most home growers wait until 8 weeks, or 10 for roasters. Others are usually slaughtered no later than 25 weeks old, and even then, they'll be tough if you don't cook them properly.
ShellyB posted this link recently,
http://www.google.com/url?sa=t&sour...6IDZDQ&usg=AFQjCNGYDKv7dSFSe9PKxmxEaWbPE8PdgA ,it's a good read. Very helpful. Other than that, try doing a forum search on the subject of cooking older birds, or tough birds, you'll find a lot of useful info if you look.