In the Manga, Pin states how the age 20 is still a kid, so I think it's safe to assume that he is around 26.
Kind of realized how unreasonable this is, so after rewatching a couple Season 1 episodes, I found something interesting.
Pin says how he doesnt have any interest in kids 10 years younger than him to Kurumi, a 15 year-old. Following season 2, he'd have to be 26, and in the manga, maybe 27.
really? great research you worked on out there!
Well, in chapter 117, when talking about ages with Ayane, she said in 10 years he'll be 37 so I assume he's 27 in their 3rd year.
He's a decade older than the main cast, so in the first season, he's 24-25. In the second season, he's 26. By the finale, he's 27, based on chapter 117 saying that in another decade, he'd be 37.
What do you think?