Fwiiw, I have walked around Tokyo pretty extensively with women that young and/or who looked that young and never been stopped by the police or even had them give us a second glance, and I am quite a bit older than my 40s. So it isn't exactly inevitable, though I guess it could happen. I don't know what they think, but in some cases they probably assume she is a young relative of mine...although in some cases that isn't so plausible (though not impossible) due to differences in our races/ethnicities.
-Ww