Hagen studied at the Norwegian Academy of Music, and has written instrumental works, electro-acoustic pieces as well as music for film and theatre.
2003 saw Hagen bestowed with the Arne Nordheim’s Composer Prize and in 2004 he received the Edvard Prize in the contemporary music category. He received a 2013 Spellemannprisen Award in the Composer category for a release with the Oslo Philharmonic, Rolf Gupta and Gjermund Larsen featuring his works. Hagen has also served as artistic director for Ny Musikk (the Norwegian section of the ISCM) and festivals Happy Days, Nordic Music Days and the Ultima Oslo Contemporary Music Festival. Hagen was appointed as permanent artistic director for the Ultima Festival in 2012, after having held the position on a temporary basis in 2009 and 2010.