Light bulbs and lamps do get darker over time; this is an effect of age and what the "hours of life" label on a light bulb is actually talking about. If a light bulb is labelled as having 2,000 hours of life, it means that after 2,000 hours it no longer gives off as much light as it used to (in fact, about 70%).
As light bulbs get older, different components fail at different rates. Filaments may become brittle and the light may flicker or dim intermittently. Other factors that can cause your light to dim or flicker include circuit overload, faulty wiring, problems with the mains supply or faulty ballasts.