Not to hijack the thread but I had a question along similar lines. I have a short AAOA since I got my first card jan this year. My wife has a credit card that she opened six years back.
Can she make her creditcard account a joint account with me and will that showup on my history and increase the average age . We have similar FICO credit scores (750+).

I am an authorized user on a couple of her cards but that doesn't seem to help. The reason I am asking is I was denied for the explorer card due to short history.

Thanks for the help.