Because Ash and West are immortaly, obviously they dont age.
But Ash likes to drop in every two years to say hello to everyone, and even after the Avatar has died and Korra's erra begins, Ash and Katara are still close friends.
Ash values the ones she loves closely, so she loves to drop in a see her god sons and daughters, Tenzin, Bumi, Kya, Iroh, ect.
Tenzin prizes Ash as much as he does his own mother, because when he was little he had a crush on her. Plus apart from his father Ash was the only other Airbender in the world.