I'm 28, and was surprised by Amnesia. I play 30m-1h every night because of tension. Still didn't end Amnesia because of this. When I - Daniel - was in some nice, but scary, rooms with more light I played more time, maybe 2h, but now I am in the prison... It's sure scary, so last night I ended only the first, upper level of it.
When I start the game every night the first moments are - hoho, nothing can scare me! It's only a game
But then I start to fear about going to every corner or room, fear the unknown hiding in the darkness, and the sound isn't helping... Then even more - I start to run like hell when seeing something disturbing. Ultimately I stop the game, it's too much for one night.
(offtop - the big plus of Amnesia is, that I can run it on my ancient PC - Athlon XP 2,4 with geforce 7600gs, and it runs and looks very good!)