I think the question of "should I start when they're younger, and then time skip, or should I start when they're older, and then have flashbacks or references to what happened before" -- that's the same question you ask for any character: should we know their backstory now, or should it come up later?
The best way to answer that question is to look at how the backstory changes the story, and the audience's impression of the characters. Here's some examples of when to start with backstory and when to save it, and why!
Be super careful about prologues!! Starting out with a "HERE'S THE WORLD THAT THE CHARACTERS LIVE IN" is very tough to pull off well -- it can be done, but a lot of times it's easier for readers to understand/get invested the world and the setting by watching the characters interact in it. A lot of creators feel an impulse to start out by explaining their world, but I think that sometimes it's not actually information that we need to know right off the bat, and sometimes it's better to see the characters in the world first, instead!
I can't say for sure if that's true for your story or not, but it's something to think about when you're planning -- how much of this information is information we NEED to know from the beginning?
But yeah, essentially I feel like, working with an older character isn't going to be necessarily any different from working with a younger character, in terms of how it's best to set up the story. It's the same question as if you had a highschool character and asked, "I have a character who's a senior in highschool.... should I show his freshman year first, or should we flashback to his freshman year later?" It depends entirely on what you want to show from his freshman year and how that information would impact your story! Same goes for an older character.
