No, there is no international standard, these policies are entirely up to the airline. As rough guidelines:
For “unaccompanied minors”, meaning traveling alone but with support from airline staff (usually a paid service), most airlines draw the line at 5 or 6 years. However, many impose additional restrictions if there are connecting flights, especially to other airlines.
From (around) 12 years, minors may be permitted to fly entirely solo, with a guardian’s permission.
In addition, policies for international flights are typically stricter than for domestic flights. As a typical example, here’s the policy from Qantas.
Credit:stackoverflow.com‘