One could argue that wet dreams are the body's way of releasing built up sperms and sexual energy.

For myself... if I let myself get to wet dream stage I start having dreams about people I know and I don't like that at all. So I try and avoid them by masturbating... it kinda helps.

Where do these things come from?
I guess at the end of the day I'd have to vote on that they are a product of your own thoughts and feelings.
I believe that God does speak to us in dreams... I even had a wet dream once in which I believe God was warning me of the consequence of having sex with a kid I was attracted to... the dream frightened me... cause as well as getting me off it had dreadful consequences for the boy concerned and made me extra cautious.... but generally I don't think wet dreams come from God specifically.

I don't believe Satan can give us dreams. Perhaps he can throw temptation at us while we are awake and that can infuluence our dreams... but I don't see any Scripture saying Satan can directly make us dream certian things (I am open to be corrected on that).

Are they the old man?... Again, I think that is a battle to be fought during your waking hours. You can't choose your dreams but maybe you can discipline your waking mind so that your dreams don't have stuff to feed off when you're asleep. Having said that, I still have constant dreams about boys myself and many of them are sexual.

Blessings
Cat.