'More than ten years old' is a good measure, but not always perfect. It's really subjective, sometimes - an obscure game might become 'old' much more quickly than a classic; some titles just date faster than others. FFVII is ten years old now, but isn't widely regarded as an 'old' game. Syphon Filter is a little newer than FFVII, but is probably considered a bit long in the tooth (even though it's still a fantastic game).
Whether or not a game's 'old' can probably depend in part on if it's still got an active presence in the collective imagination of gamers in general.