I reckon at about 3 months you can start introducing them to grass and vegetables, but very very slowly, and only one by one, as mentioned before. I introduced my Pippin to grass and a few veggies when he was about 8weeks old and he had no problems, but then again it is different with every bun.
When you do start introducing them, do so one at a time; you need to know which (if any) give him gas/runny poops etc. and so you can know to limit this or cut it out completely. If he has a run outside, make sure you only put him on it for small periods of time, perhaps just 5 minutes one day, and do this for a few days, slowly increasing over days and weeks, otherwise it's likely he will gorge on the grass straight away. Large amounts of spring grass to a rabbit not used to it has been known to cause bloat, and any swift introduction of a food can upset a bun's stomach, so you just need to be careful.
Here are some safe veggies websites that I have found very useful:
http://www.houserabbit.co.uk/resources/content/info-sheets/safefoods.htm
http://www.adoptarabbit.com/articles/packet/abcvegi.html
There are some veggies you have to be careful with and limit to perhaps only a few meals a week. Carrots are an example, due to their high sugar content. Another example is broccoli, which can cause gas if given in high quantities, but mine have never had any problem with it.
When you do start introducing veggies, watch their poops very carefully over the next few days; if you see any change then stop the veggy you are introducing.
Jen