Please offer reasoning below to the contrary, but I can't think of a valid reason for why you'd want to hide the fact that you are 18 or older.
Conversely, I can think of reasons a bad actor might wish to hide that they were an adult in order to fit in better with groups they shouldn't be involved with.
To that end, I believe that once you recieve verification, it should be a permanent feature on your profile unless someone can think of a valid reason for why this information should be hidden, this also means that Verified 18+ users shouldn't recieve the middle age verified without an age. Feel free to offer any thoughts!