They are indeed in shops owned by users under the age of 13, presumably a default thing.
If the account was created when the user was under 13, and they open a shop and they have these numbers, they don't automatically change to any other default greeting or anything once the user turns 13. Which is somewhat amusing to me if the owner of the shop never changes those description fields after they've turned 13, because it'll still be like that on accounts where the user is like 20 or something.