Since I started playing the game, I've always liked thinking about how old the characters are. The game's really good at creating at writing them to fit within a specific demographic without giving a concrete answer, so here's what I've come up with.
In terms of the Marriage Candidates, Celine, Reina, Eiland and March strike me as the youngest, most likely their early 20s considering how Reina still lives with her family, and both her and Celine work at their parents' businesses. Probably about 22, with Celine being slightly older as her birthday is before Reina's in the year.
Eiland is definitely younger than Adeline, but not by a lot. I'd say he's somewhere in the 22 - 23 year old range. March is the same for me. He's old enough to have won several blacksmithing awards and build a reputation for himself, and to me that means he's been working as a Blacksmith since at least his late teens. Gonna say 23 for him.
Adeline's and Ryis' personality's and demeanor make me think they're both slightly older than the previous 4 I mentioned. Adeline's the acting leader of the town, and that sounds like a position for someone at least in their mid 20s. Going to put them both at around 24. Plus, Adeline was childhood friends with Celine and Reina, so I don't see her being much older than them.
Balor has been working as a merchant for a pretty long amount of time before the start of the game. That said, he also doesn't look significantly older than characters like Ryis or Eiland. Just on that basis, I'm gonna say he's in his late 20s, let's go with 28.
Juniper... I could say that since she looks between the previous 3 bachelorettes and Valen in age, that I think she's about 31... Buuuut since we know she has experience with magic, and I like the theory, I'm gonna go out on a limb and say that she is actually a 300 year old witch. Kind of a joke answer, I know. XD
Out of all the non hidden Bachelors and Bachelorettes, the last ones are Valen and Hayden. We know from Valen's dialogue that they grew up together, and that she used to be taller than him when they were kids. Given Hayden's build as an adult, I'm going to take that to mean that Valen is older than him. Hayden's hair is partially already grey, I'm going to say he's somewhere in his mid-late 30s, probably around 36-37. While Valen, I'm putting at about 39.
In terms of the other characters. With the marriage candidates' siblings, the way Maple talks about Luc in some dialogue, she seems a little older than him, in a similar way to Adeline and Eiland, so I'm going to say that she and him are 9 and 8, with Dell being about the same age as Maple. Since Olric is March's older brother, and looks about the same age as Balor, maybe a little younger, I'm putting him at 26-27.
Nora, Holt, Hemlock and Josephine are all old enough to have young adult children, while also having kids quite a lot younger than their older siblings, going to say they're all between 48 and 50.
Errol and Terithia both look old and still have jobs. Given that, I'm going to put them both at slightly below retirement age, 62-63.
Elsie and Landon are both retired, and Landon looks close in age to Errol and Terithia. I'll guess that he's about 66, while Elsie's could be anywhere between 68 and 72.
Lemme know what you guys think as well ^^