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That would significantly affect your AAoA. The ideal AAoA is over 6 years...the older the better, but 6 being the threshhold of beginning to be in the "high achiever" zone.
You may want to be added as an authorized user (AU) rather than as a joint account holder.
Going from sole to joint account may result in a hard pull on both of your CRs. Also make sure that, when they resiissue the new account, it retains all of the history of the old account.
You can always back out of AU status, but cant back out as a joint account holder.