First thing first, everyone here does realize they could use the time they are spending discussing this matter to study Japanese right?
On another note, Chubby, studying multiple languages just takes time and effort. Everything with languages just takes time and effort. I was once studying Spanish, French, and Japanese simultaneously prior to my exchange to Japan. Since then my interest in Spanish culture has diminished, and as such no longer maintain my Spanish, but I still am studying French and Japanese at the same time. In short, learning multiple languages at the same time is not something worry about. You can confuse words at times, but if you work at it enough you won't have that problem (I once spoke french on a Spanish oral confusing the heck out of the other person testing, but yeah... life goes on.)
I have been told by Japanese people that I am fluent in their spoken language, but I don't consider myself fluent until I feel that I can properly express everything I would like to express with the appropriate level of softness/formality. I have only been studying Japanese for 2 years and 4 months. 7 months of this was while I was in an extremely rural part of Japan, but at the same time those 2 years and 4 months have been a lot of effort and time put into my studies.
No matter how old/young you are, it all comes down to your time and effort. While I understand busy schedules may limit time, but if you are really serious about studying the language you need to have the effort.
In the US, while I am studying French, whenever I am asked to look up French Vocabulary I also look up the Japanese word if I don't know it already.
I could also turn it around and look up french words to whatever Japanese word I look up as well.
The point is, no matter who you are, how your brain works, or whatever other EXCUSE you may have for difficulties... it all comes down to how much effort you are going to put forward. I am not saying that every person can attain a level of fluency that convinces Japanese people as such in 2 years and 4 months, but what I am saying is if you keep working at it, you will obtain that before you know it. Don't over worry about how long it takes you to learn, just learn.
Thank you for your time,