The meaning of the term "salad days" is in some dispute. A quick browse finds a number of opinions, with the predominant one that if you are in your salad days, you are "green" and therefore not quite with it yet.Its origin is well known.
From Shakespeare's Anthony and Cleopatra, 1606:CLEOPATRA: My salad days,
When I was green in judgment: cold in blood,
To say as I said then! But, come, away;
Note that she does say she was "green in judgment", supporting the naivete argument.
But my notion of what the phrase meant even for Shakespeare is that it is a reference to the wonderful time in early spring when one can first have fresh greens for a salad. It is a fleeting moment, heady and happy, when those tender delicate leaves reach their edible stage and before they begin to age to something still edible but no longer as incredibly sweet and delicious. We have gotten somewhat used to such tender greens now that they are grown commercially and cut, bagged, and readily available (though I no longer buy them). But consider that with the natural seasonal cycles in place, this lasts only six to eight weeks in the spring. It is a perfect metaphor for heedless, happy youth, a bloom that soon fades to the disillusion of experience.
Our salad days are not quite over though it is now late June (I began the post earlier). The "Merveille des Quatres Saisons" is gone, as are the arugula and spinach, but a fresh wave of "batavians" are almost ready to eat, and the "Reine des Glaces" is still green and crisp. Now I've planted my old reliable Black-seeded Simpson, a good hot weather survivor. Still, before long we'll be eating cooked greens, not that tender fleeting lettuce.