Reporting accounts on the credit reports of authorized users is allowed by Federal law, despite the fact that authorized users are not legally liable for the debts. A good way to use your positive credit to help improve your fiance's credit profile would be to add him as an authorized user on your credit cards. Many credit card companies report the card payment histories on the credit reports of both the primary cardholder and any authorized users. Making your fiance an authorized user will allow your positive credit lines to appear on his credit report and hopefully improve his credit score.
Regarding the question of adding a user to the oldest account, it doesn't really make a difference because it is going to count towards your fiance's credit from the point that you add him as an authorized user, therefore it is important that you add him as a user on the credit card accounts that will report the activity of the additional user on their credit report. Before deciding the credit cards to which you wish to add your fiance's name, I encourage you to contact the card issuers to discuss their policies regarding credit reporting for authorized users to make sure you choose card companies that will report the account histories on your fiance's credit reports.
If you would like to learn more about credit, credit reporting, and credit scores, I invite you to visit the Bills.com Credit Help page.
I wish you the best of luck in you and your fiance's efforts to rebuild his credit rating.
I hope that the information I have provided helps you Find. Learn. Save.