사람은 늙다 or 사람은 늙는다?

Answer: 사람은 늙는다.

늙다 is a verb that means grow old, not an adjective that means be old, so you cannot use 사람 늙다 to say, "He is old"; you have to use the past tense form of the verb and say, 사람 늙었다, which literally translates as, "He has grown old." 사람 늙는다 is the present tense form of the verb and translates as, "He is growing old."

젊다, on the other hand, is an adjective that means be young, so since it is an adjective, you write 사람은 젊다, not 사람은 젊었다, to mean, "He is young."

늙은 사람 (past tense verb form): a man who has grown old

젊은 사람 (present tense adjective form): a young man