One day I was going through the junk drawer in my workshop and discovered a small cowbell I had had for years. I could tell why I came up with this hair brained idea but it would take to long.
I got a leather thong and looped it through the eye on top of the bell and I hung it on the door knob of the back door which leads out to the back yard. It hanged down about 9 inches. I then proceeded to teach my dog to ring the bell when she wanted to go outside to do her business. She was a rescue dog; half Australian shepherd and half lab; about 2 years old and pretty bright. Thus began the trial and error.
I learned that hand signal only take 3-5 repetitions for a dog to get it; verbal commands about 70 reps. So anytime she went up to the door to go out, I would say "Ring the bell" then I would ring the bell and let her out. She finally got it after about four months. I'd about given up. when I wasn't expecting it; she did it.
When friends would visit; she would ring the bell; they were amazed. I was amazed.
Well after 12 years of ringing the bell, she died. I went out and got a very nice German Shepherd Dog pup; six weeks old. You guessed it: it was ring the bell time. The bell hadn't moved.
Using the above method, the young pup pulled it off in 4 days. So yes you can teach an older dog new tricks, but it's a lot quicker with a young pup.