Here is a thread from 2012 for an experience with AA - I happened to know the OP of that old post who was barely 15 at that time:
Admirals Club Access policy for underage AA elites.
Based on this report, even back in 2012 AA agents were checking the age of the guests.
However, in NRT the same 15 year old managed to visit all OW lounges by himself with no problems.