The age and breed difference doesn’t matter. I have had a lot of success bonding rabbits using an x-pen. That puts them in close quarters, but they still can move away from each other. My husband and I get in the x-pen with them, in case we need to break up any fighting (some scuffling is normal) or to stop any excessive humping. You could also give each of them a treat while you are in there, so that they associate the new rabbit with something good.

Rabbits can hurt each other if they fight. You just have to read their body language and if it looks like things are getting too serious, you would need to separate them, and try again later. There are some times when a bond just doesn’t work, and the rabbits have to be kept separately forever, but that is rare. I have bonded 5 pairs of rabbits over the years and there was never any fighting. Maybe a little chasing, growling or humping, but nothing violent at all.

There is a good bonding article here on Binky Bunny, in the BUNNY INFO link. And another good article can be found here:
http://rabbit.org/faq-bonding-multiple-rabbits/. Some articles may say to wait 2 weeks after a spay or neuter to introduce rabbits, but it would be better to wait about a month for all the hormones to die down.