In my experience AU can get your foot in the door with subprime lenders. My son started with no credit score, CapOne had denied him for an unsecured credit card. Add 2x AU's on capone cards a few month later he got a pre-approved offer for a CapOne non-rewards card, approved for $500 CL. card.
AU's are ignored by most prime lenders internal scoring systems. But may have some benefit for your FICO score but a high fico score with a thin file won't get you approved.
AFAIK lenders won't pull an AUs credit report to add as an AU. The only exception to this may be AMEX, as I've heard their AU system operates a little bit differently. An AU account will most definitely improve your credit score, if the account you (or someone else) become(s) an AU on has an entirely positive history, and if the age of that account is greater than the new AU's AAoA.
Remember when you (or someone else) become(s) an AU, the entire history or the account is inherited. Any late payments will be added to the new AU's credit report as well as the full positive history of the account.
As far as the effect on AAoA, if it is greater than the new AU's AAoA then the person's AAoA will go up, and if it is less than the new AU's AAoA their AAoA will go down (possibly resulting in a credit score drop)
Your credit will not be affected if you add an AU to your account, only theirs will be (as the account is already on your report).
If your history with the account is good, and free of any negatives, and the account is older than their average age of accounts, their score will go up.
If your history with the account is riddled with late payments, and you add them as an AU, their score will go down because of all the lates on the account.
When you are added as an AU, you don't get the card holders full credit report, just the history of the one card you are an AU on. If the card is 3 years old you gain 3 years of history as it shows on the person account, they could have every other card maxed out, it doesn't matter.
No problem. Sorry it took me so long to explain haha.
This thread may be of some use to you if you end up being a myFICO lurker like all the newcomers end up being. It has all the most common abbreviations in it that are used on the forum. I've read through that thing so many times and there's still a few I don't always pick up on immediately.