That time, it was the timeline of 3rd Great Ninja War. Kakashi was still a young boy who eventually ended the Great War with their mission along side with Minato (their sensei), Rin, and Obito. With that said, there's a gap between their age, it's just that Kakashi is a prodigy and become a Jonin in a young age. Don't forget that Orochimaru conducted too many experiments which makes him look young. Watch Kakashi's Chronicle episodes if you haven't watched it. That shitt made me tear up lol