I was in grade school and, while walking home with friends one day, we found a weathered magazine with small photos of women in various stages of undress -- mostly showing breasts, though there were a few "bushes" peeking out from unzipped jeans. It was not a high-quality publication, like Playboy or Penthouse. The photos seemed amateur.
Our local Minute Mart sold Playboy and Playgirl. That was back in the day before they sealed them inside plastic. My little brother and I would snag whatever we could reach quickly and conceal within another magazine and then sneak to the back of store to take a quick peek before tossing it behind a stand or rack to hide it before someone caught us.
The first time I ever saw penetration depicted in any form was when I was briefly living with a friend before I moved into another apartment. I came in from work one day and she, her boyfriend, his friend, and his girlfriend were watching some rented porn on her television. I just caught a brief glimpse of an up close shot as I passed by. I was 22.5, believe it or not, and had just lost my virginity a few months previous. Everything I'd seen in pornographic images, up to that point, was just nudity.
I don't know if it influenced my tastes in porn, but I certainly love up close shots of penetration! I especially love POV.