Is it normal i saw my uncle still gets morning wood?

My dad's younger brother stayed in our house for a couple of nights, and I let him use my room. My uncle's about 38 years old, but kinda looks young for his age. He's a marathon runner, so he's pretty fit and his body is in very good shape.
There was one morning when I got up early to go to school and remembered I left my books in my room so I had to go get them. I figured my uncle was still asleep so I didn't bother knocking. I opened the door and saw that he was still asleep on my bed so I just went inside, went to my desk and got my stuff.

I was about to go when I looked over at my uncle, he was asleep on his back, he was only wearing boxer-briefs and that the blanket was not covering him. I didn't notice right away, but then I realized that my uncle had an erection, and it was sticking out the front hole of his boxers. I think he forgot to button his shorts the night before, so I pretty much saw his entire erect penis pointing all the way up. Anyway, I just grabbed my stuff and got the hell out of my room, trying not to wake him. So guys in their late thirties still get morning wood?