Do I need to replace hydraulic fluid in my brakes? I've had my bike (GT Avalanche 1) for maybe four or five years now.  I've changed the pads once, removed and cleaned all parts on the bike a few times now, but I've never touched the brake fluid.  Does this need to be changed, and if so, why and how?

I couldn't honestly say whether performance has deteriorated over time - they still stop the bike and I don't notice any fade - I'm just curious.

Brakes are Tektro Auriga Comp.
Mineral oil based Hydraulic fluid will build up water condensation in the hoses over time. How much time depends on a lot of factors, like humidity, temperature, bike storage conditions, and similar things.

After 5 years, I would expect that the level of contamination would be high enough to be noticeable, and that the fluid should be replaced. The specific instructions for that brake are found here.