How can you tell how old Yu-Gi-Oh cards are?

Answer 1

Find a list of the sets, and match the card's code up to it. All cards have a code in the following format XXX or XXXX to mark the set, then a territory marker, and finally XXX for the card's position in the set.

If your card has E, A or nothing at all for the language marker, then that's one indication of an old card, cards like this are pre-Invasion of Chaos, so over 5 years old.