slides in here, hello!!
If we want to go the childhood friends route, Minato just turned 22 this month so he'll be a year older than Junpei, but they could have either known each other 1) right after Minato's and Minako's parents died and he lived with his grandparents in Japan from the age of 6-14; or 2) some high school in a different city in Japan after he runs away from his grandparent's house, ages 15-18.
Minato moved from Japan to the US when he was 19, and moved to Recollé this past November, just as reference. He moved a lot haha I tried to simplify things here.

and we can build things up from there :D