If I'm playing Modern Format, can I use an Eighth Edition Air Elemental?
Eighth Edition cards are legal in Modern.
Air Elemental is legal in Modern, no matter what set it's from, because it's found in a legal set.
While this isn't relevant to Air Elemental (since its text never changed), keep in mind that the text printed on the card is irrelevant; one always uses the Oracle text and other information found in Gatherer to determine the characteristics of a card.
- Subject to the usual requirements.