I think the translation quality is in the gutter when it's not the official so I'll trust the numbers and nothing else. Larissa is 13, he is 6 years older, Marcia is unknown.
Maybe. ╮(￣▽￣)╭
I think she is "in her yearly twentys" korean age so something like 18 or 19 same as the duke then she is supposed to already be debuted and married sinse in this world a fiance is decided for their parents and MC thought it was normal for the crow prince whose son is only 2 already be interested in finding a fiance, so I think like in real word the marriage happens soon after the debut (that is also when the ones without a fiance find pretendents are courted and childhood engagements are officialized or broke off, this means the old days teenage time, time to go to parties, meet boys, date, study to manage your future house etc)
Also while childhood engagement are usually long for obvious reasons a official engagement or someone who debuted and already found a partner has no reason to delay the marriage so it should happen max one or two years after the debut someone who doesn't even have a fiance at the age of 18/19 is past the marriage age but not soo out of the margin for a case like a broke engagement or someone who couldn't find a partner, but at this moment all good debutant nobles would be taken and this person would be left or with other person in the situations above or a divorced or widow
One year before her soul mix with OG sister her fiance broke the engagement so she who was supposed to be preparing a wedding one year ago, if debut happens at 16 she wold be max 18 one year ago (considered that engagement who doesn't go well can drag a little) she shouldn't be past 21
At first Marcia is supposed to be in her early twenties (as it was mentioned that she's past her marriageable age) then it said she was a "foolish teenager" pre-soul exchange.
Then the duke is said to be older than her however they mentioned he is 6 years older than Larissa who is 13? So HE should be 19.
WHICH ONE IS IT??! (╯°Д °)╯╧╧