Peter Graham, a Brit expat, writes, in his book Mourjou: The Life and Food of an Auvergne Village, calls for a coq "at least two years old.". Richard Olney, in The French Menu Cookbook, suggests a cock that is between 10 and 12 months of age.
IMO, a really good Coq au Vin cannot be made with a whole, typical supermarket frying chicken. The breast will dry out unless added later in the cooking process, and, in that case, the meat will not cook long enough in the wine to fully develop a good flavor. If you are relegated to a supermarket chicken - or, for that matter, most any young fryer, try using only thighs and drumsticks. They have more flavor and, since all the meat requires the same cooking time, the chicken will have a better balanced flavor and marination.
A fine option is to go to a good poultry store and get a stewing hen if you can't get an older coq.
I like Olney's method best of the several that I've tried, but Graham's method as described in his book is quite good as well. Ripert's method leaves much to be desired.