Nice idea but it's inconsistent. After initial setup I refreshed a reddit page and the age wasn't showing initially. I kept refreshing a reddit page, until i clicked the three dots on the right side of the post, then the age started showing. After rebooting the browser i tried it again and the age isn't showing, so i'm not sure what is causing the age to show next the username.

One suggestion to make this addon more useful is to add Karma details next to the age.