Do I need to replace hydraulic fluid in my brakes?
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.