- 나중에 => later; at a later time (no practical limit to the time span). Broadly used with a somewhat vague sense. - 이따가 => in a little while (limited to several hours in the same day). Colloquial term only used in conversation. Also, 이따가 can't be used in the past tense while 나중에 can. - 어려서부터 과학을 좋아하던 아이는 나중에 훌륭한 물리학자가 되었다. (이따가 won't work here) => The boy who loved science from a young age became a fine physicist later.

이따가 = 조금 있다가 = after a while = later TODAY 이따가 저녁때 만나는게 어떨까? How about meeting later today, at evening? 나중에 = at a later time [triggered by an event] 아직 2년 정도 남았는데 나중에 계약 만료 시점에 다시 연락 주세요. I'm on contract for about two years yet. Please contact me later around the time it expires. 나중에 without an event specified means "postponing indefinitely until necessary"