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Both ages as separate interviews, since his answers will be different and you will have to handle his responses differently. In the second interview, he can even look back and say "Yeah, I remember...
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#4: Attribution notice removed
Source: https://writers.stackexchange.com/a/21740 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#3: Attribution notice added
Source: https://writers.stackexchange.com/a/21740 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
Both ages as separate interviews, since his answers will be different and you will have to handle his responses differently. In the second interview, he can even look back and say "Yeah, I remember the first time you asked me this. I can't believe I said X! Now I know thus-and-such and it's _definitely_ Y."