Sorry, but can't tell you where to buy them (I bought one last minute at some ticket store in Shinujuku <-1/3 of the original price *-* 1 1/2 hours before the concert started

) and the other day one in front of the concert hall!
Most depends on the Hall where the concert takes place.
The one I went to was the Nippon Budokan and it was divided into two Entrances (depending on your seat). If you can't understand japanese this might be a bit difficult to figure out.
Btw the people standing were kept in serveral "cages" (no, not divided, really like in cages) and there was a big gap between the stage and them -> i was really happy to have a seat.
What i dislike about japanese concerts is that they take place so early... The ones I went to started at 6:30pm and ended at 8:30
If you've got some questions please feel free to ask
And by the way, you're a foreigner... that's why japanese people don't care about you're age!
I'm 18 and went to clubs (20=<) without anyone asking for my age!