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How old is the CO card and what would it drop his AAoA and AoOA to if he were removed as an AU from it?
If the baddies currently on his report get removed and the only baddies left are on the AU account, it makes sense to get rid of the AU account to obtain a clean file IMO. That's more meaningful than the benefits of an increased age of accounts sector of the FICO pie, which is less than half the size of the payment history slice.
Since he has older closed accounts on his credit report that will fall off a few years from now or longer, I'd remove him from AU on that card. 30D and 60D lates can hurt -- up to 90 points in some isolated cases.
Once he's removed, you may need to give it 1-2 cycles to fall off his reports and if they don't, dispute the account and it'll be removed.
I tend to agree - derogs trump AAoA....