I just have to add that that anybody can compose a song if you accept that a song is not excluded to a piece of music which has been formally arranged, produced, and recorded. A child can make up a song on the spot, if you can go outside the aforementioned definition. With a little help, you can play it on a piano with single keys, and sing to it. Is that not a song?
I also guess that tone deaf people can compose a song. Beethoven was deaf altogether.
Having had theory and harmony class back in high school, since I played in a few bands back during that time, it is theoretically possible to know the scale of any given key and write notes to sheet music, staying within the major or minor notes of that scale, and know the song will come out with a sound which makes sense. Of course, some songs also include going outside those rules where dissonance is actually adding to the meaning of the song, depending on what you want to achieve.