One school of thought places an age or date point on the varieties. That is, some will say that it must be over 100 years old, others 50 years, and others pick an arbitrary of 1945 which marks the end of World War II and roughly the beginning of industrial agriculture. It was after this point in history that hybrid seeds began to proliferate in the commercial seed trade.

Another way of defining heirloom varieties is to use the definition of the word "heirloom" in its truest sense. That would mean that a true heirloom is a variety that has been nurtured, selected, and handed down from one family member to another for many generations.

Additionally, there are another category of varieties that could be classified as "commercial heirlooms," that is, varieties that were introduced many generations ago and were of such merit that they have been saved, maintained and handed down - even if the seed company has gone out of business or otherwise dropped the line. Additionally, many old commercial releases have actually been family heirlooms that a seed company obtained and introduced.

Regardless of a person's specific interpretation, most authorities agree that heirlooms, by definition, must be open-pollinated and bred and stabilized using classic breeding practices. It is currently generally agreed that no genetically modified organisms can be considered heirloom varieties.

Hope this helps.