Service schedules on virtually all modern cars recommend changing the brake fluid at intervals of 2 (or sometimes 3) years. This is because brake fluid is hygroscopic, which means that it absorbs moisture from the atmosphere.
The boiling point of the brake fluid will therefore decrease over a period of time. Heat generated during braking may result in vapour forming in the system and (because vapour, unlike fluid, is compressible) may cause loss of pedal pressure or even brake failure.
Changing the brake fluid is a straightforward and inexpensive procedure which will help to maintain the efficiency of your brakes.