Yellowing of the oldest leaves of a plant at the base is typical aging. The plant drops the oldest leaves and puts its resouces into growing the new ones. As long as it's only the oldest leaves doing this and the plant otherwise looks healthy I wouldn't worry about it.
If the plant looks pale overall as well, then it could be lack of nitrogen. Houseplants don't typically need a lot of fertilizer but may need a little once in a while if they start to show symptoms. Low nitrogen causes the oldest leaves to yellow and drop because the plant recycles the nutrient to fuel the newer growth. Do you ever fertilize it?
