You saw this on "Who wants to be a millionare" didn't you?! As I thought the same when the question came up... I think salad days are because things are young and fresh like yor good days.
Actually, no, I didn't! (I haven't seen "millionaire for months). It came up in conversation a few days ago and it just seemed to be a strange turn of phrase. I thought that salad days meant that people eat more salads when the weather is hot, therefore to say "salad days" was to signify when things seem better, I suppose what we now call the feelgood factor which seems to be prevailent during those lovely sunny spells we sometimes get.