How does a brushed motor's current change over its lifetime?
The current taken by the motor will be the (torque required to turn the load at the speed the motor is running at) divided by (the field strength of the motor).

Temperature will affect the motor in several ways. Assuming ferrite magnets, their output drops slightly with temperature, 0.2%/degreeC for one grade I've looked up. This will increase current slightly with temperature. However, the windings will increase in resistance (0.4%/degreeC assuming copper), which will reduce the speed, which may reduce the load torque required, reducing the current. Either may win for your particular motor and load. However, the effect with temperature is expected to be small.

The motor bearings will be expected to wear, and as a result, friction should rise, increasing the current drawn. The effect may be small compared to the load, and may be difficult to detect, right up to bearing seizure.

While wear on the commutator and brushes will affect speed and therefore load torque slightly, the largest variable in the conductivity of that system is the surface condition of the commutator segments, which can be very different from each other without indicating any problem. The wear here is unlikely to be detectable reliably compared to the load and the normal variability, until it is catastrophic.