Answers
I'm just guessing but are you talking about being an authorized user on someone elses cards?
If so, then yes it is legal and done all of the time.
Be sure that the card(s) you are added to has excellent history (no lates, overlimit etc) and has low utilization.
When you are added you can request that they report on your reports. You "will" get all of the history of the account on your reports. It is a very good way to boost your scores.
Off topic:
I looked through your other questions to see if you or a question answerer had made any comments about being an authorized user, so I could be sure that is what you are asking.
Though I didn't see anything, I did notice one thing in your question about if you pay can they re-age. (which they cannot do)
I thought I'd add in here a few things about that question (hope you don't mind)
From your previous question you mentioned contacting the FTC. Filing complaints with them really does not result in the FTC doing anything - unless they have a kazillion complaints - but it does help put pressure on the credit bureaus and the collector that you are dealing with when you refile a dispute with the credit bureaus and include the copy of your complaint to the FTC. (the bureaus and collectors usually bend a little easier if you have a ligit complaint filed with the FTC)
Having a copy of a complaint to your states attorney general and the collectors states AG also helps in dealing with a collector that is violating your rights.
Also, the FTC required date for the reporting SOL on credit cards starts from the first time you became 30 days late and never brought the account current leading to the charge off - that date is generally 6 months earlier than the reported date of last activity (DOLA) by the "original creditor".
Your credit score affects everything from receiving the best interest rates, getting approved for loans and even getting hired for a new job. So ...
I'm not sure "safe" is the correct question.
Yes, it is a legal way to "game" the credit scoring system.
Whether that is smart is a separate issue.
Make sure you get a written guarantee.
7 Myths About Credit Repair: Real Facts to Improve Credit Scores ...
Myths about credit repair are propagated almost everywhere, from private individuals, collection agencies and debt collectors, debt settlement companies, credit counseling agencies and even the credit bureaus themselves. Knowing the real facts is vital for anyone considering credit repair.
Myth #1 - Accurate Negative Information Must Remain for 7 Years- Even if an account accurately reflects financial behavior that could be considered derogatory, such as a charge-off or collection account, that information must be reported in a certain way. In addition, according to section 611 of The Fair Credit Reporting Act the company that is reporting must be able to prove the accuracy of the information to the credit bureaus, lest a consumer has the right to request that it be removed.
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