an idiomatic expression, referring to a youthful time, accompanied by the inexperience, enthusiasm, idealism, innocence, or indiscretion that one associates with a young person. More modern use, especially in the United States, refers to a person's heyday when somebody was at the peak of his/her abilities—not necessarily in that person's youth.
The phrase was coined in Shakespeare's Antony and Cleopatra in 1606. In the speech at the end of Act One in which Cleopatra is regretting her youthful dalliances with Julius Caesar she says:
"...My salad days, / When I was green in judgment, cold in blood..."