Do old fluorescent light bulbs use less electricity?
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