HS Freshman, beginning of the school year, so 13.  I had discovered D&D and Gamma World at school, and talked my parents into letting me buy GWvol1.  My cousins came over for a weekend and we tried playing a completely-random Gamma World scenario.  We found out that none of us were that good at on-the-fly descriptions.
As a player, same time frame, I created a Brontosaurus character that I never decided if he was man-sized or Brontosaurus-sized.  To get away from some raiders, I stepped into the river and submerged briefly, then came up next round and used Radiated Eyes on the lead bandit.  (Microwaves were a new invention back then.)  The DM then noticed the entry for the Herkel ('piranha') and said to me "Guess what you stepped into?"
My first D&D session, I borrowed a friend's character as we explored some ruined temple.  We found an altar to Thor; my character made a reverence but he threw his hammer at me - a hit - down to 2HP!  The other fellow in the group says "I disbelieve in you" and Thor disappears.  I wasn't sure the group would invite me back...