It is best to start them now, because they are at the stage of just "tasting" things. By the time they are really eating a lot, their gut flora will have had time to develop the proper bacteria to handle the greens.
I would suggest holding off on the brassicas for a bit- cabbage, mustard greens, kale, kohlrabi, broccoli, and cauliflower can all cause gas. If you do feed them, give very small amounts.
I do give my rabbits wild mustard, and have not had any problems, but thought I had better mention the possibility.
yup mine start on greens soon as they start nibbling as long as its warm seasons. i always put cut forage in with the buns so about 2wks old their eyes open and they start gettin around and start eating. sometimes all in the same day!