6 is getting up there but isn't necessarily too old. I had a rabbit spayed at 7 and she came through alright, but her recovery took a bit longer than a younger buns. There is more risk at that age and as Laura mentioned, it would be good to get a blood panel done to make sure organ function is normal, as well as making sure you find a very good rabbit vet to do the surgery. Spaying would also help remove the risk of her possibly developing uterine cancer.
If you don't feel comfortable taking the risk because of her age, you could try getting him neutered and see if it will be possible to bond them. Her being hormonal could make it difficult though, as the hormones can make females more territorial and aggressive, but that can depend on the rabbit. It is possible though. I have a pair that the female wasn't spayed for the first few years of their bond, and they were fine together, but he is also a very easy going bun and not much bothers him.
After getting your boy bun neutered, you will need to wait 6 weeks for his hormones to fade before attempting to bond though, as the excessive hormonal behavior could lead to fights breaking out. Plus if you don't get her spayed, I don't know if she would be past the age of pregnancy being possible, and that is certainly something you wouldn't want to risk.