[WOL] the man was only twenty-five years old, he looked twenty years older due to excessive sun exposure.

“Although” introduces a contrast: the man was only twenty-five, yet he looked much older. The sentence could be read as “Although the man was only twenty-five years old, he looked twenty years older due to excessive sun exposure.” “Therefore” suggests a result, while “however” does not fit smoothly at the start of this dependent clause.

AI-written and checked against the verified answer.