For those who aren't sure about their ages, here's what I found:
Park Do-jin is 36 at start of the story and at its end, 38.
Kim Hye-sung is 20 at the start of the story and 22 at its end. Which means they have a 16 years age gap. (Also I love Do-jin so much, he's really thoughtful 🪐✌🏼)