Following the end of World War II and moving into the 1950s, the world economy expanded rapidly. Demand for apples and apple-related products boomed, and breeding programs brought new varieties of apples on to the market. Apple growers in Australia, New Zealand, Japan, North America and Europe competed for public favor in the fresh fruit market. The demand for trees of the new varieties pushed many of the old-time favorites out of the retail nurseries and garden centers.
Knowing about when a tree was planted is one of the clues to help identify the variety. If an apple tree in a yard or garden seems to have been planted when the house was built, and the house was constructed in the 1960s, the variety couldn't be one that was introduced in 1980. Each decade brings new varieties into the public eye, but prior to 1950 the older varieties seemed to maintain a presence and were readily available. Although new apple varieties are being developed all the time, 1950 is a particularly useful breakpoint.
In summary the likely age of an apple tree can be useful in the identification process.