Their nails tend to start being not as straight from about 5 years old - but that is a bit vague for ageing a rabbit. In younger animals, the nails / toes are nice and parallel and regular, and they grow faster. They tend to develop more irregularities as they age. Putting an actual figure to the age depends on other factors, such as genetics and how they were kept previously - but it might help to put it more towards one end of the age range you have been quoted.
Your vet and vet nurses may also be able to put a better estimate on the age from their experience. 2-6 years is an age range which basically means 'adult' - not a younger rabbit, but not 'old' either.