Nils Sigurd Aas was born in Inderøy, in Nord-Trøndelag County, Norway. He grew up in a family of prominent carpenters and furniture makers. His grandfather (also named Nils Aas) had established a carpentry workshop and water-driven sawmill in Straumen. His father, Ivar Aas, took over the carpentry workshop at the death of the elder Nils Aas in 1927, and eventually won awards for his furniture work. The family also operated a Sunday School out of the same building as the workshop, from 1933-73. Aas' mother Inga Lie was from Telemark and had come to the village in 1928, working for the local parish priest. Ivar and Inga met and married the same year, and the couple had five children: Marit (1929), Signe (1930), Nils (1933), Aslaug (1935) and Arne (1941). In his youth, Nils was an active as ski jumper and athlete.