Reliably using motor at higher than normal voltage
If the data sheet says 18v, then you can be assured that if you apply 18v, the motor will spin fast, faster than at 12v, and won't fall part or burn in that short test.

A brushed motor's brushes and commutator will wear at a rate dependent on the voltage and the operating current of the motor. These both determine how much energy is deposited in the sparks that occur at the commutator, and erode it. The size of the sparks is also determined by the commutation timing, that is the angle that the commutator electrical axis makes to the field magnetic axis. This angle is usually adjusted to be optimum at the rated voltage, and for the designed direction of rotation. 

You can assume the motor will wear faster at 18v than at 12v. It will not be (18/12) times faster, I doubt it would even be as low as (18/12)^3 times faster, but raised to some much higher power. That will be acceptable in some performance applications, and not acceptable in hi-rel applications. You must determine either by experiment, or by getting more detailed specs from the vendor, what the lifetime will be under your operating conditions, to see whether it's acceptable to you.

Note that BLDC motors do not have brushes. If you want reliable operation, then it might be worthwhile looking at these.