Budget airlines, such as Ryanair and easyJet, won't accept unaccompanied minors under 16 years of age. (There are no exceptions to this rule because they're not insured to carry unaccompanied persons under 16 years).
Full-fare carriers, such as BA or Lufthansa, will carry carry unaccompanied children (although they might impose conditions, such as a parent or guardian accompanying the child to the departure airport and/or meeting them on arrival).
Officials at airports will normally ask questions when a young person is travelling without an adult. It would be best (irrespective of any official policies) if a parent/guardian accompanied the passenger to the airport. Most overseas countries will permit unaccompanied minors to enter although some, particularly Italy, impose conditions.
If you require further information, please provide further information. e.g. ages, destination and purpose of travel (i.e. would the teenagers be travelling to visit relatives who could meet them or would they be hoping to get a hotel room?
Chris