Does a penis shrink with age?
Sure hope not! I haven't got much to shrink.
I think the simple explanations are increased fat pad and less blood to the cock as one ages. However, we're mostly talking about flaccid here; whatever shrinkage that occurs in older men is probably associated with ED of some sort.
There's a guy I used to know from the gym when I was around 20. He was at least in his 70's. he was still a good-looking man, very dapper, with a tiny pencil mustache; always wore a sports jacket hat with a feather in the band. He still had a bevy of girlfriends. And there was a big reason for that. He had the biggest cock I'd ever seen up to that time (and still, probably). It was very thick and "hung down to his knees" -- or at least it seemed to. He obviously hadn't suffered from shrinkage. And, if he had, I'd be hard pressed to imagine what it was like before. We showered together many times. I'm just the opposite; although I was much taller (6'3" to his 5'7"), mine barely peeks out of the pubic hair. He took an interest in me. He once told me the secret of his success with women. Good dental health! He still had beautiful teeth. He said your teeth and your smile were the first things a woman saw of you. I didn't dispute the point with him, but I wondered how he'd fare if we'd swapped cocks at birth.