They both indicate a time 12 years in the future BUT the way we would use them is different. "In twelve years" is the way we usually speak to discuss what will happen 12 years in the future from NOW. We usually use this with the future tense. Examples: Right now I'm single. In twelve years, I'll be married. I'm going home in 2 months. I'm almost finished work, so I'm leaving in five minutes. **If you use this form with past tense, you change the meaning from "future" to "during".** Example: In twelve years he got married, divorced, and married again. He ate his entire lunch in only ten minutes! "Twelve years later" is used from a point in time that is NOT now. You often hear this used in narration (stories, history, etc). Examples: He finished university, and two weeks later he had a job. I ate bad food for dinner last night, and three hours later I was sick. My wife is having a baby next June, and we're moving into a new house one week later. It sounds strange to use "twelve years later" if you're talking about now. Example of a BAD sentence: I'm almost finished work so I'm going home twenty minutes later. I just heard that my wife will have a baby 7 months later. So in your scenario, both are potentially possible, depending on your meaning. If you were talking to a client with a new baby, and were suggesting they start saving when he's six years old, you would say "I would make the deposit by the time he's six years old. Then, twelve years later, when he's in college ..." But if say, they had a six year old, and you were telling them to start saving now, you would say "You should make the deposit now. Then, in twelve years, when he's in college ..."