one's youthful days is correct because salad days means one's youthful or inexperienced period of life.

This idiom or phrase is useful because it helps students understand the intended meaning beyond the individual words.

Examples: In her salad days, she dreamed of becoming a singer. He made many mistakes in his salad days, but he learned from them.

The idiom refers broadly to youth and inexperience, not old age or only school days.