I've that them occasionally thought my life and strangely, they never seem to correlate to not having cum for a while. I'd be jacking off every day as a teenager and would still occasionally have a wet dream and one of my most recent times I had cum earlier that day.
I also had "dry dreams" when I was 10-11, before I had started shooting sperm - same sexual dreams, same feelings of an orgasm, but dry and no mess! And at least one was about being in a public restroom or locker room or something similar, watching men. This was long before internet and I was a pretty sheltered country kid, so how did my 10 year old mind come up with those dreams?? 🙂