Depends on the condition of the tradeline and how it reports its open date. If the tradeline has high utilization, negative history, etc then it can hurt. If the tradeline reports as opened when the person was added as an AU then it will drop AAoA. If you're automatically adding each other as AU's on new cards then you're definitely dropping each other's AAoA's. That said, AAoA is a relatively minor factor.
http://www.myfico.com/crediteducatio...yourscore.aspx

If you're looking to build credit you can't rely on being an AU and need to build your own tradelines.