There were so many posts worthy of quoting, I just decided to start a fresh one, to summarize many of the thoughts already proposed.
First, if it was too long to begin with you could be reaching the adjustment ends prematurely. Check with a parts fish for the correct number of lengths for your year (assuming it has stock diameter sprockets). Now at some point it may have been the proper length chain, but someone decreased the size of the front sprocket making the chain effectively too long. Chains often need to be shortened when a smaller front sprocket is used.
The question was asked about age. The real question is mileage. If the chain is relatively low mileage, below 3000, lets say, then even if it had been neglected it wouldn't have stretched (actually worn, chains don't stretch) that far and the scenario is one of a too long chain.
The test is, take all the slack out of the chain by pushing up on the bottom row. Then at about 3 OClock on the rear sprocket pull the link away from the sprocket. If there is no, or only a skosh of daylight showiing between the chain and the sprocket you can still use it. Remove a link (2 actually as you can't remove just one) and start over.
If you can see daylight and more, throw it away and buy new. On a budget you can skip the sprockets, just realize your new chain will wear faster on the old ones. But it will probably last long enough (3-5 thou miles) for your treasury to recover and you can replace everything.
