Gotta love how these cliches persist...
Even in the medieval ages, child marriages were rare in both orient and occident. Normally, nobles'd get engaged as children and marry as adults (which, depending on culture, would be somewhere between 14 and 21 years of age).
The only regular exceptions were orphaned girls, as they'd get the groom's family's protection through the marriage. But even then, it'd be socially expected for the marriage to be in name only until the bride became an adult. Most popular example of this is probably mohammed, who married a girl at 9 years of age, but kept her apart until she was 13. Still makes him a perverted hebephile (especially considering how old he was), but completely different from the "get married at 8 and have children at 10" narrative older history books claimed.
In Japan specifically, girls would live with the groom's family for a period of time prior to marriage, with both sides being able to annul the engagement during this period. I'd guess that's what's going on in this chapter.