Old coins are preferred over new coins because transaction spammers spend coins over-and-over again quickly to try to flood the network with transactions.
Unintentional transaction spammers (like people playing SatoshiDice over and over) accidentally do the same thing, so their transactions are given low priority and can take a long time to get confirmed.
Maybe think of it this way:
+ Transaction fees are to pay miners to include your transactions.
+ Transaction priority is used to "pay" for the network bandwith used by free transactions (note that any node relaying or listening for transactions pays the cost of every extra transaction).