I have a character that is an old man, and acts as a mentor to the younger characters. My story is going to have flashbacks to when he was a kid, in the same position as the kids he is mentoring, but I am really struggling in visioning who he was as a kid. Think Obi Wan. Mentor to Luke. Then we see what he was like as an apprentice. How do you work backwards to really figure out who someone was?
I think the only way to do this is to write his life story first, at least in outline form. You won't use most of it in the actual story, but you need to understand it before you write. Remember, if an old man were really reflecting back on his younger days, it would be through the window of all the intervening years of experience. You need to know what that experience is.
Ed's right, you have to know his background story very well. If you want to make sure that you are capturing the old man looking back and not just the younger version's view, try doing a short writing exercise where you first write a flashback scene from the young man's POV and then write it from the old man's POV. That will help you focus on the elements that convey that filter of age and experience. (Or maybe your goal is to write the flashbacks from the younger man's perspective in order to contrast how he is today from how he used to be, or to establish connection with the younger characters he is mentoring?) Either way, the exercise can help you differentiate.
I agree with what those above me have said. I also think that its important to realize that a lot of people lie about or idealize their past. Have the character firmly conceived as an older man, and you can make his memories an any way he would perceive them.
