It's really difficult to tell once they have reached adulthood. There are some signs that a rabbit may be a little older rather than younger, for instance my vet says she never sees horizontal ridges on incisors (a sign of dental disease) in rabbits less than a year old. Teeth also discolour with age, but also with diet. So there are a few different indications but nothing very concrete, so it would only ever be a best guess. You can also make inferences based on where they've come from - statistically most rabbits which are given up by their owners are done so in the first year of life, so you could use that as a proxy. I have a feeling that most insurers will accept a best guess (this is certainly what I've done in the past without problem), ideally based on the best guess of your vet as at least that gives you some kind of 'evidence' as such, but I don't think that's necessary and I've never been queried on it.