To be designated as such, the thing should have been around long enough for a consensus to develop among those qualified to judge.
Classics have lasting value.
Turn on the radio to hear music that has endured for decades or even centuries, with listeners never tiring of them. These are the classics. Look on the road for cars that have been lovingly maintained, or restored to original condition and monetary value often many times the original purchase price. These are the classics. Visit the Louvre in Paris to see huge crowds still admiring cracked and faded paintings done by the great masters of previous centuries.