i'm new to rabbits myself but breeding age for rabbits seems to be 6-8 months old. some say to start at 6 months is better. others say wait until 8 months. some have other ages that they wait for. besides age i would say, no matter the species, look at the animal and make sure they are big enough and mature enough. with rabbits common saying seems to be "8 months or 8 pounds" and with goats its "8 months or 80 pounds" though personally with goats i like to wait until a year. that way i know they wont have stunted growth, less possible birthing issues, etc. also depends on what breed you have.
just make sure you find the right age for you and your animals.
For small sized rabbits I heard you can breed them at 5 months because the kits will come a month later when they are 6 months old. Medium sized rabbits I would wait till they are 6 months old to breed. And for large breeds like Flemish giants, you should wait till they are 8 or 9 months old since it takes awhile longer for them to mature.
I agree with flemish, from what I've read smaller breeds can be bred earlier. The 8lbs wouldn't really make sense because alot of rabbit breeds don't ever get that big. I have mini lops and they're only 4 1/2 - 6 lbs at a year and a half and might reach 7 1/2 in their later years. Netherland Dwarfs shouldn't get over 3 lbs.