"Salad days" is a Shakespearean idiomatic expression that means a youthful time, a period of carefree innocence, idealism and pleasure associated with youth. The modern use, especially in the United States, refers to a heyday, a period when somebody was at the peak of their abilities, not necessarily in their youth.
Queen Elizabeth II used the phrase during her Silver Jubilee royal address in 1977, referring to her vow to God and her people when she made her 21st birthday broadcast: "Although that vow was made in my salad days, when I was green in judgment, I do not regret nor retract one word of it."
Here is her words of the vow:
I declare before you all that my whole life. Whether it be long or short,shall be devoted to your service, to our great imperial family, to which we all belong.
Here is her image and voice: