In practice, I think most computer fans can take a lot more than their rated voltage due to the simple nature of their circuits. I have a friend who pushed a computer fan to over 25v before it blew up. As for longevity, I broke the original fan that came with my printer, so I got a new 50mm fan rated for 12v. It's been working fine for a few years now.
I did notice that the motor of the 30mm hotend fan got rather toasty when given the full 19v, so I wired a few 330 ohm resistors into the fan lead to drop the voltage and thus the heat.
What I'm really worried about is h