I met him at a Worldwide D&D Game Day a couple years ago, and he signed my "25th Anniversery D&D" book. He looked so old, so frail; his skin was like thin white tissue paper. I figured he was in his 70s or 80s.
When I got home I looked him up on Wikipedia and was shocked to learn he was in his 50s. The next day at work I asked my boss (who was a gamer and the same age as Arneson, and had met him at the same event), "How old do you think Dave Arneson is?" He replied, "Oh, I dunno. Maybe 75?" When I told him they were the same age, he was a little freaked-out.