The thing about retrievers, is they keep on learning. They want to "DO" for you. They want to achieve. If you don't train them, they will likely have their own ideas and eat your couch. Old or young, they want to do for you.
Training for a Labrador should start as soon as they cross the threshold. I have links on this.
In my case, learning to live with a Golden retriever is easy. They will do whatever you want, but mostly want to be loved. Will do what is necessary for that. Adapt to weird hotel rooms, be kind to "weird" people in wheelchairs or on walkers.. Or live in a different environment. I intend to take Em to Colorado, where mostly you have to stay on leash, but sometimes you can swim.