The idiomatic expression Salad Days originally referred to a period of youthful inexperience, naivety, immaturity, impetuousness, or unskillful exuberance. Like many food-related expressions, the term was coined by William Shakespeare. Specifically, the term was used by Cleopatra in the play Antony and Cleopatra, meaning the expression stretches back to 1606.
The beautiful Egyptian queen Cleopatra, in the play, is very much in love with Marc Antony. But, her servant, Charmian, points out that she once expressed the same intensity of emotion for Julius Caesar.
Cleopatra replied jokingly that those were her salad days when she was “green in judgment, cold in blood.” She was explaining that in her youth she didn’t know as much about the ways of love.
Salad days, then, is a play on the association between “greenness” and youthfulness, which was nothing new when Shakespeare coined the term. There were already earlier phrases that coupled the color green with inexperience.
Greenhead meant an inexperienced fool. Greenhorn referred to (and still does) a newly enlisted soldier or any sort of novice.
The meaning of the expression salad days has changed a bit since Shakespeare first used it. Whereas before it had the positive connotation of youthful happiness and enthusiasm coupled with youthful foolishness, inexperience, and misjudgment, now, the term is just as likely to be wholly positive, referring to when someone is not only happy, but when times are at their best and a person is at the peak of their success. With this usage, youth is not necessarily required.