You can say .. 갑돌이와 갑순이는 어렸을 때 부터 사귀어왔다. We can change " They have been the couple since they were young" to " 그들은 어렸을 때 부터 만나왔어 ( or 사겼었어 ) " / "사귀다, 만나다(verb)" = couple I hope you get it.

"GapDol and GapSoon have been a couple since they were young." => 갑돌이와 갑순이는 어렸을 때부터 커플이었어요. (because of -부터, 이었어요 is understood to be "have been") => 갑돌이와 갑순이는 어릴 때부터 커플로 지내 왔어요. (to say "have been ..." more explicitly - but not really necessary) The verb for "be" in English is 이다 (technically called a particle), which is sort of special as it's the only one that directly attaches to the noun. - 갑돌이와 을돌이는 형제다: Gapdol and Euldol are brothers. (이다 contracts to 다 after a syllable ending with a vowel) - 갑돌이와 갑순이는 남매가 아니다: Gapdol and Gapsoon are not brorther and sister. (아니다 is the negation of 이다) - 그와 나는 10년 전부터 친구였다 = ... 10년 전부터 친구로 지냈다 [지내 왔다]: He and I have been friends since 10 years ago.