Interesting question for me as a playwright. I've written plays with the issue of a younger and an older version of the same character. I think you're right about the gestures and the mannerisms. In my case, the same actors both played the older and younger versions of themselves - about a 25-year difference. The audiences seemed to buy that with no problem based on the feedback that I received.
You have to remember that people so change some as they grow older. (Trying to recognize your high school classmates at a 20-year reunion can be a real challenge.) As long as there aren't really big differences in body type or size, I think audiences suspend disbelief pretty well.
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Jean Klein
Playwriting Teacher in MFA program, Wilkes University]
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