It all depends on the dog actually. If the dog is still able to run and jump and do the kind of things that need to be done in agility without any real danger to it, it should be okay to go for it. And 12 year old dogs can still compete they just end up in the veterens catagory which means that they get to jump 4" lower then what they are supposed to which means the dog can do agility for longer, but I might suggest you get in contact with a school and see what they suggest, otherwise it's up to you and your dog.