Well, it depends on how much dimmer we're taking about. The CFL tube that most laptops use for backlighting will dim over time, but if its seriously dark, then it may be a failing backlight rather than normal age-related dimming.
Consider comparing it with a newer machine and posting a pic, or even taking it to an Apple genius.
My daughter uses a 1st gen MacBook with a lot of hours on it and its not really dim at all for a machine of its age.