Hi there!
The only time one may consider not to fill a cavity is when the tooth is rendered poor or hopeless meaning that it's not worth saving. So with that rule, we should always try to arrest the decay activity of any cavities. Arresting the decay activity could be having fillings done or could even be as simple as close monitoring at home and with the dentist with an improvement of oral and dietary habits.
That being said, adult teeth eruption occurs as early as 4.5 years old to as late as 15 years old. If the tooth shows signs of mobility or shedding, one may discuss with their dentist the option of leaving the cavity alone.
Hope this helps!
Cheers,
Kenneth
