Salad days' is used these days to refer to the days of carefree innocence and pleasure of our youth. It has also been used to refer to the time of material affluence in our more mature years, when the pressures of life have begun to ease - something akin to 'the golden years'. Shakespeare meant the former, and the clue is in the colour. While he used green in other contexts to signify jealousy - 'the green-eyed monster' in Othello and, in Love's Labours Lost "Green indeed is the colour of lovers", it is used here to mean immature. The green of salad leaves, which are invariably short-lived, is an obvious allusion to youthfulness. Green is also used in other expressions to mean unready for use, for example, 'green (unripe) corn', 'green (unseasoned) timber and 'greenhorn' (an inexperienced recruit).
The phrase 'salad days' lay dormant for two hundred years or more but became used widely in the 19th century. For example, this citation from the Oregon newspaper The Morning Oregonian, J