I was 8 years old, and I had never played an M rated game before. So, what does my friend do? He puts it on The Maw, Legendary, gives me a pistol and a shotgun and tells me to sit in a corner and kill anything that moves while he goes and grenades the objectives. I had nightmares for a week.
I went back after that week and beat the whole game, co-op, on legendary, in one sitting, so I apparently liked it more than I thought I did.