As your eyes age, you will need brighter sources of light when you read, so the recommended wattage for reading is 100 watts. When purchasing a reading light for a child or young adult, you can go as low as 40 watts.
There is no perfect method for selecting the wattage, so try different types until you find one that is comfortable for you. Higher wattage produces a brighter light that helps illuminate the text on the page.