I have a Mazda 3 2010 at 100,000 miles. Unfortunately I don't have the exact history for replacement but recently had a vehicle report done at a mechanic where they started I should replace front brake pads and rotors. I could be missing it but am not noticing any braking problems? Is there any objective way to know when brake pads/rotors need to be replaced.
If the calipers are in good condition and the slide pins are not restricted then the pads should wear evenly. If they do not wear evenly then your brakes will become unsafe sooner due to excessive wear on one side.
The service manuals for vehicles give you a minimum rotor thickness and pad thickness. If the rotors get to thin they warp and that can increase stopping distance. If your pads are really low and you brake hard so all the material is now gone, you are metal on metal. This again longer brake distance. It is all about safety.
It's therefore best to replace brake components whilst they are working well, so you can always stop.
The thicknesses depend on vehicle and is in the service manuals.