I’ve heard about baby rabbits not being ready for greens because of their new digestive system...so I was wondering...what age can they start to eat greens?
Generally speaking, it is suggested to wait until bunny is 12 weeks of age. At that age, one can slowly introduce greens -- one type at a time and just a little bit to start. This page explains how to safely introduce greens.
If bunny starts to get runny or mushy poo, then stop feeding that particular green. If it happens with several greens, then hold off a few weeks before trying again.
The exception to this advise if it is known absolutely that baby had already been fed greens along with its momma who also regularly received greens. In these instances, baby's gut has developed the enzymes needed to digest those greens.