I would not mind if the age of the account started on the day that the AU was added. This is the only category that doesn't show the if the AU is credit worthy, the advantage the AU would have over getting his/her own credit is no HP and guaranteed approval (as long as the card reports AUs). IMO, being an AU is similar to being a joint account holder. You both take care of the account and won't want high Util. or a missed payment to affect you, having the entire age of the account just as if it were yours all is not an accurate measure of being credit worthy (of course, I will use it if it is there, but won't miss it).
However, after a review of the legalities of it, FICO backed off and reverse their decision and re-included AUs on the newest FICO versions. The Equal Credit Opportunity Act was interpreted to prohibit the exclusion of AUs and it was that law that led to the reversal.
Authorized User Accounts (everyday accounts). Wasn't it about spouses? FICO "later reversed their decision after consumer advocates observed that FICO 08 could unfairly impact women and/or stay-at-home spouses, and lenders’ concerns that using the new model would impede their compliance under Regulation B—which requires lenders to consider credit histories on accounts shared by spouses when assessing a married person’s credit risk." FICO had actually found a way to separate such valid AUs from the rest, so I don't know why they felt compelled to toss the whole deal; the article touches on that too. I hope that lawmakers at any potential time would acknowledge the distinction.