Easy. Your voice wavers just a little bit when you talk, like it didn't break all that long ago. And you're clearly still growing - your physical proportions are slightly off, like most teenagers. [Most people wouldn't notice something like that. Most people wouldn't think to try and notice something like that. But Heizou? He's always been able to pick up on that kind of thing.]

So... I'd say you're sixteen, at the most.

[He looks across the table expectantly, waiting to be confirmed as correct.]

As for my name, I've never liked to go by my family name outside of work. It sounds too much like I'm in trouble when people use it.