Learn languages young, because people are boring.

Learn languages young, because people are too fucking boring to bear once you have any judgment: advice I never got.

Learning languages to be able to communicate with a people is the province of naive youth.  Most people suck, in your own language and in others'.  Once you have the judgment of adulthood, if you do, you narrow who you spend time with, because everyone else is a waste of time for you.  Categorize your friends right now.  How many are not: your class, your gender, your race, are interested in your hobbies, do the same work as you.  Sure, few friends match all of these, but I bet they all match three of the five.  You could add more categories (politics, religion...) but the point is that your friends match the majority.

Remember youth when you could spend hours talking shit with anyone who'd listen to you; when, in fact, that is all you did with your time in high school and in college?  That was the time to talk in a second language to build your vocabulary and fluency.  It's too late now.  Sure, the mind is more elastic when you are younger, especially with language, but my guess is that the more relevant factor is the sheer amount of language input and output that young people do.  After all, which ESL students have you seen progress most quickly?  The ones who won't shut up, not the wall-flowers; the Brazilians, not the Japanese; the women, not the men...

It's the ten-thousand hours of practice meme (despise Gladwell, but it's a useful meme): whatever latent abilities you do, or do not, have, the more practice you get the better you will become, and more quickly.  There is just about no way you can recreate the amount of free time yourself, much less from others your adult age, that you had when you were much younger; nor can you recreate the oral/aurally intense environment of schooling and socializing.  You need to work, and will only be paid anything decent when working in your own language, and adults do not go out of their way looking for friendships with people they cannot communicate with so you can practice their language with them.  However, you're an English speaker reading a text-rich blog: good luck finding educated, interesting people who do not speak English better than you speak their language.  It happens, but not often.  You'll be speaking in English, or you'll be speaking with uninteresting people most often.

You still want to learn fluent conversational Japanese, or whatever language?  Three words:

       fuck - romantic relationship(s) in only the target language
      drink - your alcohol loosened tongue and their alcohol lubricated patience with your ineptitude
      play - a shared interest or hobby with target language speakers so there is some reason for them
       to tolerate the tediousness of communicating with you

All of these are much less available if you are married, have children, and/or are recovering from addiction or bad relationships.  Sounds like adulthood to me.

You can learn to read and write without social interactions, if you want to know the language like the Japanese know English...