At age 13 I flew alone, not registered as unaccompanied, on multiple domestic flights in the US with no issues, never questioned, no eyebrows raised.
At age 13 I flew alone, not registered as unaccompanied, internationally to/from Australia and Europe from the USA, no issues, never questioned.
From then, aged 14 - 18, I flew dozens of international flights and many many more domestic flights all without incident.