[Ocean's Edge]

I *think* perhaps the water test is whether there is some OTHER attribute as well as being old..

Whether it's a particularly fine example of an important architectural style, or the who owned / built it, or significant event(s) took place there...

We own a 120 yr old house - it is old, it is not however historically significant - or at least only minorly so. It having been owned and built by one of the community's original prominent families - but it's a small community, and it's built in an unusual and distinctive look for the area, but not built by the well known architect of the area for that period - although he too was a member of the same extended family. It's more an interesting curiosity than a landmark.