There's no reason to add a minor as an AU except in the case of Amex, where it reports the open date as the date the AU was added.
If you add someone the month before they turn 18, the will inherit the exact same history as they would doing it years earlier.
IMO they are much more likely to be the victim of ID theft if there is a prime credit profile just sitting there unmonitored.
If you feel the need to add minors as an AU, which IMO is not worth it, freeze their credit reports.