The sad realisation that the greatest thing you’ll do or accomplish in your life time was done at the age of 12, making the rest of your life seemingly meaningless. Making a monument for a person of when they were a child even though they lived to a ripe old age is kind of insulting.
Certainly, to the rest of the world he peaked at twelve. I’m sure Aang saw things differently. He lived a life free of further conflict, raised three children, and loved his wife. Far less dramatic than saving the world, but no less meaningful, and no less important to making it worth living in.
Plus, I couldn’t imagine a greater reward for someone who spent so much of his youth bearing such a heavy load.