For starters, I rather like this definition from Wikipedia:
An heirloom plant...is a cultivar that was commonly grown during earlier periods in human history, but which is not used in modern large-scale agriculture.
Still, I don't think this quite nails it. At least not for apples, which scale pretty well compared to, say, wheat. Granny Smith is, to my mind, an heirloom due to her history and her age, yet you can find Granny year 'round in most supermarkets thanks to modern agriculture.
Suppose the rare and wonderful Westfield Seek-No-Further, by some miracle of market forces, were sold in supermarkets across North America. It would still be an heirloom, and no less wonderful (if no longer rare).
Defining as heirlooms or antiques only those things that are hard to find would be a kind of snobbery.
Some writers periodize agriculture based on the advent of the refrigerated railroad boxcar (though they do not always agree about when that was). Anything that originated before that is an heirloom.
For Lee Calhoun, apple maven of the South, the threshold year is 1928, when he says groceries supplanted farms as the primary source for fruit.
These intriguing definitions capture a little of the complexity of how human tastes, technology, and apples co-evolve together.
Such distinctions are really qualitative, not quantitative. "Heirloom" describes something of history and human society, not just biology. It expresses a social relationship.
Suzanne Long tweets that she prefers "the term 'heritage apple' over anything else in current use."
This is an elegant usage that I have sometimes emulated (and, according to Wikipedia, is Australian). The "heritage" quality lives not so much in the apple as in its relationship to human history.
Unfortunately it's not always so useful a part of speech as the other sobriquets. When every word counts, it's handy to be able to write about antiques and heirlooms. However if I ask a farmer if he or she has any "heritages" I am going to get a very strange look.