I first saw one, I think, when I was about four. I have this one unconfirmed memory of my mom taking me into the bathroom and explaining to me what a penis was while my dad was toweling off. I shit you not. This has been a vivid "memory" of mine for decades, but I hope I'm imagining or mis-remembering it. And I don't want to have a verifying discussion with my mom ever, so I will probably never know what actually happened.
About a year or two later, my parents did make me watch a VHS with Henry Winkler. The creep in overalls strumming about penises haunted me vaguely until a few months ago, when I realized I could simply Google "henry winkler child molestation" and find the specifics.
When I was about seven, my aunt and uncle gave me a copy of The Anatomy Coloring Book along with a set of fucking sweet magic markers. I'm pretty sure the male genitalia page was the first to get Marvy-ed.
Basically, my fascination with dicks began early, long before I found out that babies didn't just "happen" to women once they got married. I didn't see one in its proper state until I was 17, but it wasn't much of a surprise because of the following fact: I discovered my dad's porn stash* when I was 11. On the outside, I was a quiet and studious adolescent with an autistic level of interest in Mozart. But being an only child of two working parents allowed me to spend my summer days paying excruciating attention to every minute of my dad's VHSes, each of which was like 8 hours long, crammed with back-to-back pornos from the 1970s and early '80s. It was the same scientific-inquiry skill set that had me winning state science fairs. And yes, I knew who Ron Jeremy was long before anyone else in my age group did.
As for your memory, Middy, I have a few questions. Did he whip it out without warning or explanation? Did he fiddle with it? While (in theory only) I admire his spirit of adventure, I wonder why he chose to show it to you in its revolting, flopsy** baseline. Samey for the solidly middle-class-looking man who whipped it out for me on a deserted side street in Chartres when I was backpacking solo in France. I was already pretty familiar with dicks by that point, but it was extra startling and pathetic-looking for being uncircumcised. I walked straight to the train station from there.
* We're talking boxes and boxes of mags here, and about a dozen videos.
** Best word choice ever, Middy. Bravo.