This scoring factor considers the average age of your accounts as well as the ages of your oldest and newest trade lines. (creditcards.com)
Also, canceling a credit card won't hurt your average age of account as long as it's not your oldest card. (johnnyjet.com)
So by holding onto the oldest cards it provides a better average age of accounts for all those new rewards cards I like to open. (pointsmilesandmore.com)