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The story came up when the interviewer asked Arin what she wanted to do once she finally turned 20 years old. (Arin is currently 18 years old in Korean age, 17 internationally.) She said, "There are unnies in the group who have driver's licenses. I want to go driving with them in their cars. And... once I turn 20 years old, I want to have serious talks with the unnies." 

The other members then went on to poke fun at Arin, going, "Drinking? Drinking," before Seunghee had to explain, "We've talked about how it would be fun if when Arin finally turned 20 years old, we can all have a drink and have a serious talk." 

Binnie then added, "We promised not to drink alcohol until Arin turned of age. Even when there's a dinner at the company, we don't drink." Arin expressed her apologies to the members, but they reasoned that if Arin gets older, it means they get older too!

That's true!