An authorized user can spend on a card account without being responsible for the debt. The reporting consequences of that arrangement are larger than the spending permission suggests.
Two different relationships to one account
The primary account holder applied for the credit, agreed to the terms and owes the balance. The issuer can pursue them for the full amount.
An authorized user is granted a card on the account and can transact, but has no contractual obligation to repay. The issuer generally cannot collect from them.
A joint account holder is a third arrangement entirely, where both parties applied and both are liable. The three are frequently confused and behave differently.
The account history can appear on both files
Many issuers report the account to the credit bureaus under the authorized user's name as well as the primary holder's, including its age, limit and payment record.
Because the whole history is reported, an authorized user added to a long-established account may see that account's full age reflected on their file.
Not every issuer reports authorized users, and reporting practice varies, so whether the arrangement shows up at all depends on the specific issuer.
The transfer of history runs in both directions
A well-managed account can strengthen a thin file, which is the usual reason a parent adds a young adult to a card.
The same mechanism transmits late payments and high utilization, and the authorized user has no ability to prevent either since they do not control the account.
That asymmetry is the substance of the arrangement: the user takes on reporting consequences without gaining any control over what is reported.
Removal is possible but not retroactive by default
Either party can usually ask the issuer to remove an authorized user, and the account then stops being reported to that person going forward.
Whether the historical entry disappears from the file depends on the issuer and the bureau, and removal may take one or more reporting cycles to appear.
Anyone relying on removal to resolve a problem should verify the change on their own file rather than assuming the request was completed.
Lenders can see more than the score reflects
A tradeline reported as an authorized user is typically flagged as such in the underlying file, even where scoring models include it.
Manual underwriting can therefore discount an account the applicant does not owe, particularly where it accounts for most of the file's depth.
The arrangement builds file history rather than borrowing history, and the two are treated differently once a human reviews an application.