According to the official PlayStation website, only some PlayStation Network codes have expiration dates. Since expiration dates on PSN codes vary depending on the type of code, owners should check their codes to ensure that they are still valid.
If a users experiences trouble using a PSN code purchased from a retailer, he should contact the store of purchase to ensure the card is activated. Sony is unable to help gamers activate their PSN codes because only the retailer that sold the card can activate a card.
Players can only redeem PSN codes in the same region in which they purchased them. They can also only redeem them on their Sony Entertainment Network Master accounts. They cannot redeem PSN codes on sub-accounts.