The answer was accepted, but a remark: Pay attention to the accents. With the suggested correction on "létech" instead of "letech" which was in the word bank. "Když se vrátil po dvou létech, vypadal mnohem starší."
Interestingly, "léto" (summer) has two possible forms in locative plural: "letech", which is normally used when we mean "years", and also "létech" - which sounds more, at least to me, as if you literally said "after two summers".
Když se vrátil po dvou letech vypadal daleko starší. What is wrong: the sequence of "se vrátil po dvou letech" instead of "se po dvou letech vrátil", or is daleko starší not accepted as much older or both?
Nothing is wrong with either, both are fine. It wasn't accepted because nobody had thought of adding "daleko" as an option. Literally, it means "far older", but the meaning is the same, so I've added "daleko" now.