I remember my mom telling me that when i was 7 (that's when i got my n64 with super mario 64), she woke up in the middle of the night to find my father sitting in front of the TV playing the game in the "Lethal Lava Land" level, turning to her, and saying "I'm in hell!"
I can't get over how funny that is in retrospect, but for the time (and for me even now) seeing that hellish environment with volcanoes, lava, and horned bowling balls, all in 3D, was amazing, and in no way exclusively made for kids.
Yes, mario might cater to younger audiences, but that doesn't mean we can't have some fun with it, right?