Even if you were 20, you would HAVE to have prepay, there is no way you could get a contract for a number of reasons.
Keep looking, maybe there are Japanese people that would help you get access to a prepay once you get there, maybe have a good friend buy it for you and put it under their name, or something.
Another Idea (which is a lot spottier, but possibly could work out well) Is to buy an ipod touch. It would most def. be many times cheaper than any kind of cell phone deal, and most japanese people prefer to email on their phones because of high airtime rates. You would just have to find a wireless network once in a while, or stay in one while communicating.
There are many maps online that show free hotspots in major cities, and warwalkers/drivers that provide info on locations of free but not so legal hotspots.
When I went to japan for a month I rented a cell phone. It was kind of expensive, but it was nice to have one. I used JCR, renting a phone for 5 months would be very expensive, but if you really needed one... it would be $750 for just rental, extra for the minutes. (option A)
Jcrcorp.com