There are many variables that impact the life of an irrigation system. Exposure to freeze/thaw cycles, the type of soil surrounding the pipe, quality of the water and the intensity of use of the system are just a few factors that must be considered. Another consideration is the advancements in irrigation technology. While the pipe may still be good, the control system may need to be updated to capitalize on improvements in water and energy management that can save thousands of dollars per year, or even per month, depending on the cost of water and power in your area.
The only sure way to determine if the irrigation system on your course is due replacement is to employ an irrigation engineer to perform a system analysis. As a very general recommendation, it is time to start investigating the need for replacing the entire irrigation system once it has reached 15 years of age. The control system should be evaluated at least every 10 years to determine if advances in technology justify an upgrade.