Use the adjective old to describe someone who's been alive for a long time. When you're 30, 60 seems old, and when you're 60, 90 seems old. When you're 90, face it, you're old!
The adjective old also means former. It can be fun to visit your favorite teachers at your old elementary school, or to drive through your old neighborhood. You can use old to describe something of long duration, like your old friends from camp who you visit every summer. Old also has numerous informal or slang uses, like the phrase "any old," which you'd use to say, "I don't want just any old job; I want to be a circus clown."