Despite the physical transformation, he is indeed supposed to be at most 12-13. And most characters still actually regard him as a teen.
Peobably in the murim world his original body is from a teen but in his original world he is already a doctor serving for the war so he's probably in his mid 20's.
From what I have seen, he looks like he's in his late teens, early twenties when by himself, but when they zoom out or when he stands next to someone, he appears to be between 14-16 in appearance. I think he is actually a little younger than that. Maybe pre-teen age 12-13 .
Chapter 77 spoiler. I guess??
His body changes. And like. He looked around 12 - 13 but now after getting bone rearrangement thingy. He looks like he's on his 20's. It has only been 3 months. How old was he supposed to be anyway (╯°Д °)╯╧╧