I just inherited an old kit from 1999. Came with light malt extract and rice malt extract that is 10 years old now. I decided to be adventurous and make a batch of beer with both of these.
I noticed the cans did not bulge on either end so I figure they would be okay. Anyway, I opened them up and decided to taste them. To my utter surprise they tasted like raisins and plums, each of them. And they were very dark in color.
Well I just harvested a bunch of wild apples and decided I would try to make an apple, cinnamon, dark brown sugar, 10 year old malt beer. It is extremely sweet and therefore I am going to have to age it another couple months but I will keep updated on how the flavor turns out when it is all said in done.
That all being said, I say it is never too old to use.