Car Starter is simply a DC motor that turns the engine crankshaft through the flywheel, starting the combustion process by creating compression within the cylinders. Voltage to the starter is supplied directly from the battery and is controlled by a relay and/or solenoid operated from the key switch inside your car.
A slow cranking engine may be a sign of a bad starter and with age,
that's very likely. It may also be due to low battery voltage, poor
electrical connections at the battery or a failed relay or fusible link.
Starting your car with the major components turned off (like the AC
compressor, blower motor and high-powered stereos) will greatly ease the
load on the starter.
Although the starter drive can be replaced separately from the starter
assembly, it is better to replace the whole starter assembly. Failure of
any part is due to age, usage and heat stress, to which the entire
starter has also been subjected. So the other parts are also just as old
and stressed. It's quite common to replace the starter drive only to buy
another starter in a few months because the brushes wore out, a magnet
broke, the solenoid failed, among other common mishaps.