They’re from the December’s Nindori.

Judging solely from their appearance, his age order for the bachelors will be something like this.

As expected, Leon is the oldest and there’s no doubt about it. He also came from the oldest time period. Dylas’ place is set as well. When it comes to Vishnal, Doug and Arthur, the order’s quite tentative and difficult to place. Arthur may be calm and knowledgeable, but he’s still lacking experiences, which explains why he’s in the same age range with Vishnal and Doug. Kiel without doubt is definitely the youngest among them. But then, he added that since the game takes place in a fantasy world, so it should be reasonable to consider it when you think for the  suitable age for marriage in this game. He gave no exact age for any of them and leave it to the player to decide. 

As for the girls, based on their appearance only, it’d be something like this. 

Forte’s the oldest among them. Also, despite her seriousness and sense of duty, she’s in fact the girliest of all. Xiao Pai and Kohaku look very young, with Kohaku looks the youngest. She may be indeed the youngest…Note the “May”. It’s important. He leaves it to the player to decide, but after you’ve seen her marriage sub-event. 

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And then, not game related but since I’m on it already…

I think they’re coming from a page from the RF4’s Tokuma Shoten/Nindori Guide Book.

Apparently Hashimoto’s most favorite character in the RF4 is Kohaku.

Meanwhile, appearance wise, the series’ character designer Iwasaki Minako is partial with the monster traces (?) in Dylas and Kohaku.