I think there was a cultural change on this sometime in the late 1990s/early 2000s. I remember when I started secondary school it was all about one-strap, but by the time I left I was definitely two-straps (so between 1998 and 2005). This 'study' agrees:

http://www.slate.com/articles/life/cool_story/2013/10/two_straps_on_a_backpack_or_one_strap_what_s_cool.single.html

How do you wear yours?