[guroot]

You just have to phrase it correctly: https://www.wolframalpha.com/input/?i=population+of+japan+ag...

It's 4.75% according to alpha based on a 2010 estimate.

[antmldr]

I think that's the 15-19 age group...

[guroot]

Ah, you're right I didn't look closely at the results. derp