He retired as a professor and department head, continuing as senior researcher until 1991.
[1][2] Hagen published a large body of work, with research of considerable depth.
Among his more notable works was Historiens røtter, released in 1982 as volume one of Cappelens verdenshistorie.
Hagen also served as leader of the Norwegian Society for the Conservation of Nature from 1963 to 1969 and Statens naturvernråd from 1970 to 1972.
He was decorated as a Knight First Class Royal Norwegian Order of St. Olav in 1989.