Here's a more important question. Since she was a working detective on the police force and the story never once had acting like a minor, why was her character a minor in the first place.
The only time it comes up is when anything sexual is about to happen, but otherwise, she is a freaking detective, it's so silly she's a minor.
The entire situation is very bizarre tbf. I don't know about japanese laws about child labor, but I'm sure a minor won't be able to work as a detective, which according to what was shown up until now, it's a dangerous profession...
Hell, I could simply say that saying that she's a minor is just a bait for those who are
into this kind of stuff, but I rather avoid bringing moralism into an literature work, even though I said previously that I was glad that nothing happened due to her age.
As others stated, even if she says that she's ready, she isn't exactly ready to take the next step, since she's pretty immature about anything not related to work. But yeah, in the end it's all about how much padding the author decides to do...