@MorrisTheCat Thank you for the link! I'll check it.

I'm not opposed to your answer or to adding to Ash's answer. I'm curious about how it would work.

The ideas are great, but I was thinking more of the Earth twin as passive. He/she would have to be surprised that the other twin is older. Perhaps I should've mentioned it in the main post.

@Hypnosifl Thank you. I would prefer it doesn't break causality, but I'm also open to a certain degree of handwavium.

@Hypnosifl Suspended animation wouldn't work for me. What I'm thinking is person A met person B while person B was a child. The child Person B spent some time "somewhere". Some years passed (say 8 years), and Person A reencounters Person B, finding out Person B isn't a child anymore, but an elderly. Perhaps this is just too far from the plausibility realm?

@Hypnosifl Yes, I'm thinking of the twin who stayed on Earth as the younger one. Is there any way it would be possible?

Thank you. And would there be a plausible way (or at least a bit handwavium) to explain someone who grew old faster in relation to an observer on Earth?