"He was a student activist in his college days."
(彼は大学時代には学生運動家だった。)
解説
「当時は」「昔は」「この頃は」「最近3年間に」「過去には」「将来は」「二十代に」「五十代で」などもみな、かなり時間的幅のある概念で、'in' を使います。「結婚生活は幸だった」は"He was happy in his married life."となります。同様に、"in those days", "in old days", "in these days", "in the last three years", "in the past", "in the future", "in my twenties", "in his fifties" となります。
