If he has any instinct (retrieving, etc.), you should be able to train him. Labs are especially eager to please, and the old saw about old dogs and new tricks is really not that accurate. Just like most other animals, they keep on learning all their lives.
It might be a little tougher if you have to break bad habits that are deeply ingrained, but certainly not impossible. Go for it!
If the dog has a desire to please you then it can work. Don't expect a pro but if he/she saves you a few trips outa the blind or stumbles across that cripple in the tules then you'll be set. Just make sure his second name isn't GET OVER HERE !!! I think about 40% of the dogs I see on refuges are named GET OVER HERE!!!!!
Listen to these guys. Instinct, desire, and trainability. Instinct can be shown by wether or not the dog has any interest in fetching, or take a dead duck out and throw it and see if he has an interest in sniffing it up and bringing it back. If he doesn't give up too easily then he has desire. If he/she is 5, then you should have taught him/her to sit/come/etc, and if that was real difficult then your dog may not be real trainable. Sounds like alot but really it isn't and it really doesn't matter if your dog is 5 years or 5 months, if they don't pass these tests then they will be a problem.