When my wife was 18 her dad added her as a joint owner on one of his credit cards that he had had since before she was born. This was mostly done so that she could use it in an emergency while in college. She used it twice ever. On her credit report, this credit card is listed and the report says that she has a credit history older than she is. Long credit history is good.
Seeing as this credit card appears on our credit report, CreditKarma lets us know exactly how much of a balance is on it (its my FIL's primary card). Unfortunately, my FIL isn't as good at paying his balance off every month and carries a multiple thousand dollar debt. Not paying it off every month is bad.
Over the past year DW and I have been working on improving her credit. The credit card joint owned with her dad and a ton of student loans were the only things she had so her credit wasn't terribly great. Over the past year, she's gotten a rewards credit card, started using it effectively and is smart about always paying it off entirely every month. We've started to see her credit increase dramatically over the past year because of this (and her being named on some bills that are always paid).
We are trying to decide if keeping her listed on the credit card with her father is a good idea. Is the benefit of a credit history older than she is greater than the negative of having a rolling balance of multiple thousands of dollars? If its a wash, we'll likely ask for her to be taken off of the card because she wants to fend for herself now.
(How we gently tell him "we want her off your card because you don't pay your balance" is a different story that we'll figure out when we get to it)