What causes the yellow light of death becaue my fatty is five years old and I think it might be on it's last legs.
A number of things could happen, but most are avoidable. Really, the only thing that would create a YLOD is misuse over time. Take care of your electronics properly with consistent attention and maitenance, and you should almost never have problems with overheating or component failure. The exception is the BRD itself, that can fail for numerous reasons that have nothing to do with how well you maintain the system.
Plus, having additional cooling sources is helpful with these units. Invest in a cheap external cooler and you'll be doing yourself a huge favor. Constantly monitor the dust and debris that could fall into the vents (I keep mine upright, so this is a concern) and overall just keep the area clean where it is stored. Do all that you can to properly shut down the unit with the touch power button and not the power source switch in the back. Every once in a while this is unavoidable.
Just like any other computer, take care of it and maintain it and it'll last a long time.