And apparently in the original Japanese translation, the FEMC still "spends a long time" with him. It's the line that a player usually gets in a social link with heavy romantic subtext that's rank 10 and the characters are in a bedroom with one another. Maybe playing an intense game of chess? In any case, the localization team added the "you spent a long time talking" but taking the original dialogue into account, it wasn’t talking they were doing.
Maybe another intense game of chess?
But the trend continues in both Persona games that follow. In P4 you can be 16-17 and enter some relationship with an older nurse. P5 has you being 16-17 and able to date your older female teacher, or your older local doctor. But a 10 year old is kind of in a different league... they're re-releasing P3 on Xbox so I guess we'll see if they'll keep this scene in.
Only time will tell if Persona 6 continues that trend.