This isn’t the age of your characters, or your account, it’s for your forum activity.
You can see it by middle clicking your profile picture at the top, to open it in a new tab with a summary of your forum character.
I’m not sure on the wording that is used corresponding to trust level 0, 1, 2 or 3, but there’s 4 ranks.
https://us.forums.blizzard.com/en/wow/t/new-forum-guide-syntax-and-features/19903/1
This guide by one of the greens pinned in General Discussion is a bit out of date so the requirements on each level might not be correct anymore however getting to Trust level one used to be…
so it won’t be too far off, post a few times, read a few threads, give a few likes and should bump you up soon.