Hmm, I also think that Nakyum might be still a teenager, probably 17 to 19, and Seungho could be about 7 years older, probably mid twenties? I don't think they're older than that, considering the era and how things would've been perhaps. Also, the old servant, it was said he's been with Seungho about 10 years? And he knows what happened to Seungho (which could've taken place at about 13-15 years old, so all of that would fit with him being in the mid twenties now. As for Nakuym, that's just a guess based on his looks, experience, relationship with the InHun and physique
Yes! I think this fanart might be quite close, though I think the age difference in there is much bigger, with Seungho being a young teen, and Nakyum being about 4? So that could give us about 9-10 year difference. Still possible though, but I do think they're closer in age than that and I think you might be right about Seungho being a bit younger than InHun

Any guesses HOW OLD our boys are, Nakyum and Seungho?