We used clicker training, and it always impressed me how even the oldest of dogs could learn new tricks (har har) if you found what motivated it, and then incentivized a specific action based on that motivation.
Even more fun was finding a really smart dog who you could teach to do the craziest stuff. Had one who probably learned 25ish tricks in his short time at the shelter. He was so motivated to learn. It was a pretty easy sell to the right adopter when you showed them you could point your finger at the dog, yell "HANDS UP" and he'd sit up with his paws in the air, you'd yell "BANG" and he'd fall over dead. How could you ever resist?
Example (not my video): https://www.youtube.com/watch?v=lky1xsqYGUo