There is not a single example of an underage marriage in the entire story. Arranged engagements yes, but not a single noble pair married, before their graduation + coming of age. In fact we have been outright told that it’s exceptionally rare for Detlinde to already marry just a single year after her gradation, when those plans were still on, due to just a single year of engagement being considered a rather rushed affair. That would have put her at 17, meaning that most nobles only marry at 18 or later. As in 18 their calender, which would be well 19 years old in a 365 day system.
If I have the numbers correctly in my head, Ferdinand is 13 years older than Myne (not counting the 2 year coma or age-shift for Rozemyne’s fake noble birth), and Benno is another 8 or so years older than Ferdinand. While 13 is a lot, 21 is still quite a bit more… At least Ferdinand isn’t old enough to comfortably be is future wife’s father, which Benno is easily.