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.
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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.