Say, for example, you have a card that is still legal in standard but the version you have of it comes from a set that has since been rotated out. Can you still play the version from your older set?
Yes. The only thing that matters for determining card legality is the name of the card. As far as the Oracle is concerned, 2 different printings of shock are the exact same card for all game and tournament rule purposes.
From the tournament rules:
3.3. Authorized Cards Players may use any Authorized Game Cards from Magic: The Gathering expansions, core sets, special sets, supplements, and promotional printings. Authorized Game Cards are cards that, unaltered, meet the following conditions:
- The card is genuine and published by Wizards of the Coast.
- The card a standard Magic back or is a double-faced card.
- The card does not have squared corners.
- The card has black or white borders.
- The card is not a token card.
- The card is not damaged or modified in a way that might make it marked.
- The card is otherwise legal for the tournament as defined by the format.
- The card is a proxy issued by the judge of a tournament (see section 3.4 for rules about proxies).
Any other cards that are not Authorized Game Cards are prohibited in all sanctioned tournaments. Unglued and Unhinged basic land cards are allowed in sanctioned Magic tournaments.
Players may use cards from the Alpha printing only if the deck is in opaque sleeves.
Players may use otherwise-legal non-English and/or misprinted cards provided they are not using them to create an advantage by using misleading text or pictures. Official promotional textless spells are allowed in sanctioned Magic tournaments in which they would otherwise be legal.
Artistic modifications are acceptable in sanctioned tournaments, provided that the modifications do not make the card art unrecognizable, contain substantial strategic advice, or contain offensive images. Artistic modifications also may not obstruct or change the mana cost or name of the card.
The Head Judge is the final authority on acceptable cards for a tournament.the mana cost or name of the card.