I've read that old fluorescent light bulbs produce less light over time.
Do they also use less electricity, or do they use the same amount, but produce more heat?
All lamp manufactures have the lamp curves, as the lamps are started it damages them a little and when running they are degraded but slowly. The industry standard is the lamp is at end of life when the light output is below 50%. Lamp life curve The ballast controls the wattage consumed this stays the same for the life of the lamp until the contacts are eroded and cannot strike. another lamp life example
I have never heard of the wattage being affected by age. Temperature yes. I had to get this information directly from a high-bay light manufacturer as it is not published. A T5HO ballast used the listed wattage when the test chamber was at 20 degrees Celsius. But when the test chamber temp was increased to 25 degrees Celsius the lumen output increased as did the wattage. This explains why so many T5 fixtures had an efficiency greater than 100%. They used the ballast wattage at the lower 20C and the fixture lumen output at the higher 25C.
The lumen loss lines up well with the dirt loss. As the lamp gets covered in dust, the light output decreases. This would not affect the wattage. At around 20 months, the light loss factor of fluorescent is about 0.9. This is also about the dirt depreciation factor at 20 months. https://ouc.bizenergyadvisor.com/BEA1/OMA/OMA_Lighting/OMA-12
There may be some change in wattage as the mercury gets embedded in the glass over time. Affecting lumen output and fixture wattage. But I think the primary culprit is dust on the lamp and fixture surfaces which won't affect the fixture wattage.
It's a waste, here's two lesser-considered reasons why:
Regardless of the actual lumens/watt of any particular old CFL, they are dimmer than new ones. That means the consumer will inevitably turn on additional lamps to achieve the desired brightness. This doubles the amount of power used for the task, even if the per-unit is the same old vs new.
Considering the electronics of the driver, things get more complicated than just phosphor wear. The capacitors used on CFLs, esp cheap CFLs are prone to failure. These are relatively high-voltage parts, which are expensive, leading manufacturers of bulbs, especially cheap ones, searching for the cheapest unit. They don't need to last 10 years, so they are MTBF-spec'd for the task.
As they dry, they lose capacity. As the capacitors lose capacity, the CFL driver must work harder to regulate output voltage. This causes more heat, and we all know where heat comes from; power. I don't have before/after measurements on like bulbs, but i know that old ones (even if completely dark) still use power, unlike a tungsten filament.