I thought it was a post-Vader Anakin. His lesson to Ahsoka, in my eyes, was that his life, her life, as soldiers led them to a "fight or die" mentality, though maybe "kill or die" is a better way to put it. That's what drove him to the dark side. Only by the end of his life, as he learns from Luke, there is another option.

"You're never too old to learn" isn't just in reference to Ahsoka's age, but in reference to this last lesson he learns from his son, that he now passes onto her.