I realy don't know, but they were hear longe befor us, fern's die down each year and re grow the next.

Nephrolepis exaltata, That must be a massive plant now unless you have been dividing it and a good age too
it's one of the oldest I have heard of but considering your location it is understandable, here in the UK it is not a hardy fern and is usually a house plant

I agree, and in answer to how old I will add that ferns can live for a very long time especially species that self divide of course as they can keep going. There may be clonal plants of many species (including flowering plants) in the wild that could be centuries old or even older, perhaps since the first colonists after the end of last ice age 10,000 years ago, the Creosote plant in south west USA being a prime and well known example. Some plants have formed steadily expanding rings of growth of a measurable size that are known to be first colonists approx 10,000 years old. I diverge of course but you get the point....

I'm guessing here that Plug may check this out :-) so.......just looked this up myself as the above was from memory.

http://en.wikipedia.org/wiki/King_Clone