The drinking age on international flights generally follows the laws of the country where the airline is registered, but there are some variations depending on the airline and the route. Here's a breakdown of how it typically works:

1. Airline's Country of Registration

• Airlines Registered in the U.S.: For airlines such as American Airlines, Delta Air Lines, or United Airlines, the drinking age is generally 21, since the U.S. has a legal drinking age of 21. This applies even if the flight is flying to or from a country where the legal drinking age is lower.

• Airlines Registered in Other Countries: For example, British Airways (UK), Air France (France), or Lufthansa (Germany), the drinking age will align with the laws of the country in which the airline is based. In many European countries, the legal drinking age is typically 18 or 16, but this can depend on the specific country and airline.

2. International Routes

• When flying internationally, the legal drinking age will also depend on the departure and arrival countries. However, the flight crew will often adhere to the rules based on the airline's country of origin or the regulations for international flights.

3. The Rules During the Flight

• On international flights, the flight attendants will serve alcohol based on the airline’s home country laws, but passengers should also be aware that they must comply with the country's drinking age laws when the plane is at the departure airport or arriving in a new country. This means that while on the flight, the legal drinking age on the plane is often based on the airline's country of registration.

4. Special Circumstances

• Connecting Flights: If you’re flying on connecting flights, the drinking age on each flight can vary depending on the airline's country of origin. For instance, if you switch between a U.S.-based airline and a European airline, the drinking age may differ.

• Duty-Free Purchases: When purchasing alcohol in duty-free shops, the drinking age is typically aligned with the country where the shop is located. For example, if you’re passing through an airport in a country with a drinking age of 18, you can purchase alcohol at the duty-free shop regardless of your age, but you must adhere to the drinking age once on board the flight.

Summary

• U.S. Airlines: Drinking age is generally 21, regardless of the destination.
• European or International Airlines: Drinking age could be 18 or 21, depending on the airline’s country of registration.
• Duty-Free Purchases: The legal drinking age depends on the airport’s location, not the airline.

For more information read this blog : What's The Drinking Age On International Flights?