age
*Act your age!
e.g.: Act your age, Jim! You're not a child any more!
Grow up, Robert! Put your feet down!
agree
*I couldn't agree more.
e.g.: A: What's the use of being paid good money if you don't spend it!
B: I couldn't agree more.
apple
*An apple a day keeps the doctor away.
*the apple of (someone's) eye
e.g.: She had another son, who was the apple of her eye.
Oh, darling, don't say so, you're the apple of my eye!
Hey, don't touch that car, it's the apple of my son's eye.