A 13 years old author can come up with jolly good ideas the same as anybody, assuming a gifted imagination, but writing a wholesome, well thought-out story with three-dimensional characters, in an appropriate manner (since this is still manga/anime), takes a bit of maturity and perspective. So, let's not judge SAO too harshly. Besides, SAO was well-received among its intended audience of other 13 years olds.