You can usually get an accurate age up until about 6 months. After that most breeds are considered adults so telling the age can be tricky. For larger breeds, you might be able to tell until about 8-10 months.
Shelters and vets might go by looking to see if the rabbit acts younger or older (older rabbit could be less active). While doing an exam, a vet might be able to find some things that could point to a rabbit being on the older side.
Pretty much once they get to be adults you can't really tell the age. You can go for a younger adult and an older rabbit, but pinpointing it beyond that is probably not going to happen.