Suzenne gives you excellent advice.
Unfortunalty you can not determan a gooses age by it's size, Believe it or not geese can range from under 1 year old to as old as 25.
I have found that the younger geese skin is softer then the older birds (uncooked). with 11 subspecies ranging from duck size ((cackling Canada) to over 20 # (giant canada) and some old birds being small and young birds being large (ugh!!) I tend to cook my goose till medium rare and only serve the breast sliced very thin on a bias.If you find you have a young tender bird that's a bonus because you can enjoy the legs,thighs and wings. But I usually make a cassorole or curry with those cuts.
Let me share with you a recipe I use at times when I am sure I have and old bugger.
1 goose cut into pieces
2 tablespoons butter
2 tablespoons grapeseed oil (or conola)
2 cups chicken stock
1 cup beef stock
or 3 cups goose stock if you have it.
2 tablespoons A1 sauce
3 tablespoons dry sherry
a little fresh margeram and thyme
Kosher salt and fresh ground black pepper
2 bay leaves
3 tablespoons slurry (water and corn starch)
1# kiln dried cherries
2 table spoons brandy.
dredge the goose in the seasoned flour and brown in the butter and oil in a dutch oven or oven proof pan,brown a few pieces at a time, deglaze with the sherry,add the stock (hot) A1,herbs S&P and bay leaves,return the goose to the pan,cover and place in a 325* oven for at least 2 1/2 hours,until tender.remove the goose pieces add your cherris,bring to a shimmer,skim any fat then add your slurry and brandy.stir over heat till slightly thickned,pop the goose back in the sauce and baste for about 15 minutes..be sure to adjust your seasoning at this time. just plate it up and serve more sauce on the side.
you don't need to use the sherry or brandy if you so desire but just scale the stock up .
Baruch ben Rueven / Chanaבראד, ילד של ריימונד והאלאן