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.
Your first statement is still correct.
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.
Rephrased to more explicitly place “for 20 years” in the earth’s reference frame.