It could be a genetic disease. I once saw a documentary about a girl who was 15 or so, but was stuck in the body (and largely, mind as well) of a baby. His could be something similar.
But honestly, from what we've seen so far, it's likely to be something his master (I shudder just thinking about that monster, one of the most despicable villains ever) did to him. Either advanced medical technique, or something mystical. It might even have something to do with the dead languages, but probably something else. After the spoon-bending, we know that there are other mystical forces in this universe besides the dead languages.