Jim,
The other professionals have pretty well answered your question while I was out this morning, but will add that I think the major life shortening element of old systems was the design velocity. Technically, PVC pipe can stand up to 6FPS, maybe 7FPS velocity, but it puts a lot of pressure in the joints. We used to design for about 5FPS, but with additions over the years, supers not breaking out the zones as we intended, etc., the overall and periodic water velocity went up.
Texas now mandates a design standard of 4.5 feet/second on irrigation systems. While I am not sure it is something that needs mandating, its a great guideline, especially if you size pipes for about 10% future additions (if not a wall to wall system)
Also, some note winters are hard on systems, but as to lifespan, southern systems also run almost twice as many months, which tends to shorten their lifespan. If your pump does anything else, like pump out lakes to lower water, the pump and that section of pipe may wear out even quicker.
But, mostly, its leaks at the joints and thrust blocks that do you in. As to your other question about replacing sprinklers, many courses have upgraded sprinklers and controls while keeping pipe, as well as extending pipe.
In a recent speech, I highlighted both the ASGCA/USGA "Life Cycle" chart, and noted that it was written by a member of the Fazio team and the USGA......both of whom seem to hang out at the upper crust clubs. I assured superintendents that where I work, many systems are made to last beyond the 25 year life expectancy noted there.
Hope this helps.