I think that linguistically speaking the term "young" means like UP TO 7 years old. At about 8-9 you are considered an adult (again, linguistically).
I personally think the reason behind this is that your own language isn't quite fully developed enough to be getting in the way of your second language yet. The goal of all language learners (who want to speak fluently) is to basically have two independent languages-- you don't want your L1 (mother tongue) influencing your L2 (second language) too much. Whether you realize it or not, that is a huge part of it.
There is a way to bypass that part of the brain that causes your mother tongue to get in the way of your L2 acquisition, but it is a very difficult process which takes some people longer than others and requires an amount of immersion (a lot of direct exposure to the L2). I've read many times that something like 3% of language learners seem to have this down from the get go, even when learning their second language as an 'adult'.
It is my opinion that English education in Japan falls WAY short of this concept. Likewise, Japanese education in America can be just as bad and was in my experience (but not standardized like English is in Japan).
Textbooks and the like tend to be written in one's mother tongue (as written language would be EXTREMELY hard to learn from immersion). The textbook approach might be neccessary very early on, but I'm beginning to wonder about that recently. Using textbooks and things like that are great for learning a lot in a short time. The problem is, people usually only get so far. At least I've noticed that certain people give up on language learning quite systematically (in high school it was the 3rd year, in college it was the 2nd/3rd year).
I think that generally expectations set by textbooks are a little bit overbearing. A native speaker of a language can't hold a conversation in 2-3 years, so I think it's a little bit much to expect an L2 learning to pick it up in a year or 2.
Immersion is a lot more taxing on the learning and takes a lot more time than textbook learning, but once something is learned it is quite solid (as it is learned by hearing something over and over until you are confident enough to use the phrase yourself). With this style of learning, you often see someone who is struggling suddenly go from really bad to being able to hold a conversation comfortably within a month or two's time span. This is not unlike a native's own acquisition. Natives usually transform, langauge wise, into a person who can barely speak, to an annoying little kid very quickly
.
So in other words, to repeat myself, "young" (linguistically speaking) is generally considered to be that small window of 0-7 or so. I personally believe that "young" (again, linguistically speaking) is a state of mind that you can train yourself into having. This also seems to account for those 2-3% of "adults" who seem to somehow "get" their L2 from the very start.