Within the European Union, unaccompanied minors under the age of 18 MAY require some authorisation when travelling between countries.
As regards minors entering Portugal see this pdf document from the website of the Embassy of Portugal in Washington, D.C.
http://portugal.usembassy.gov/uploads/Xl/ep/XlepckUL1UNRC_CaspnJZA/minors_exiting_entering.pdf
If flying, one should also check with the airline to be used as they may have additional requirements.
