I recommend introducing vegetables, fruits, and outdoor exercise time (where they can eat grass) at around 3 months of age. You can totally introduce these things sooner, if you decide to deworm your rabbit!
By 5-6 months of age, they should be fully used to all types of greens

Remember, even if you have an older rabbit, it is always important to introduce something slowly to see how they react! I like to start with herbs such as parsley and cilantro, as it is easier to limit intake by ripping off a few stems. Definitely introduce vegetables before fruit, as fruit is considered the treat and can be very sugary. Vegetables, on the other hand, provide many essential nutrients and vitamins and are considered part of their diet. A good rule of thumb is to start with a pinky size amount of vegetables and gradually increase to 1/2 - 1 cup daily! I also like to stick to organic greens as it gives me some peace of mind that my rabbits will not be eating anything harmful. You can grow it yourself, or buy it organic from your grocery store! Best of luck!