To be more explicit: What is needed to make the statement interestingly wrong is for the two 20-year figures to be in different reference frames. If your twin does something for 20 years, then they will be 20 years older; but if they do something for what you experience as 20 years they may not be.
Edited to more firmly attach "for 20 years" to the earth.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
Notes for future OT posters:
1. Please add the 'open_thread' tag.
2. Check if there is an active Open Thread before posting a new one.
3. Open Threads should be posted in Discussion, and not Main.
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