A dog is never too old to Force Fetch.
1) It has to be done properly.
2) It has to be worth it to the person force fetching the dog.
3) If you are going through the process, you have to be willing to reinforce it.
If you are unwilling or unable to do any of the above three, then just hunt him and deal with it. If it becomes that huge of an issue, then quit.
I have Force Fetched many dogs over the age of 5 years. Some readily take to it and some resist more. But its the same for dogs under the age of 1 year old too.
WRL