Brake problems on 2006 Saturn VUE?

I have a Saturn Vue 2006 AWD with 47,000 miles on it. I had my tires rotated recently and they told me that I need front brake linings and rotors. My rear brakes are OK except the mechanic said I need new rear brake cylinders as he states they are leaking. I have not seen any brake fluid anywhere, the only thing is that my brakes squeal when the weather is damp. I know that the front brakes probably need to be done, but why would the rear cylinders be leaking when I have such low mileage on the vehice? The mechanice also said I could wait for 3000-10000 more miles before I have this work done. If the brakes are leaking, shouldn't that be more urgent to repair?