If I remember correctly she was a couple years older than him, but he called her ahjumma mostly because she was a married woman. I'm not an expert on korean culture or anything so I can only speak from what I've learned in dramas and articles and so on. And as far as I know ahjumma is a term used typically to refer to a married woman, but also an older woman you are not familiar with, so the situations in which a woman is called ahjumma may vary (same with ahjussi), it's a little tricky. For instance in "flower boy next door" the guy called the girl ahjumma, but she was a 20 year-old single lady and they were about the same age. But in birth of a beauty I think he called her ahjumma because she was a married woman and there wasn't another title to call her by. (Like she called him Doctor because she initially thought he was a doctor, so in his case there was a title for him). ^^
