They have 15 years age gap.
During the flashback to when Surim's Mom and Wookyeong's Mom met,
Surim was 7 and Wookyeong was 22.
In the current timeline, Surim is 20 and Wookyeong is 34 which could mean he hasn't had his birthday yet.
During the first flashback I think in Chapter 31 where Woomin (Wookyeong's brother) was first shown, Surim wasn't around. Meaning he wasn't born yet. Since Wookyeong's brother said he was 13 like Manager Choi that time, I'm guessing Wookyeong was like 15 since he older than them.
Then it said "A few years later" which then says Wookyeong's 17 and Baby Surim who tried to steal his wallet was 2 or nearing 2 that time.

So how old are they?