Just looking over some of the previous ball-parks for teams that are now in "new" parks, it looks like 30-40 years of use is pretty normal.
Pirates (3 Rivers) - 30 years
Phillies (Veterans) - 31 years
Cardinals (Busch) - 39 years
Twins (baggie dome) - 27 years
Orioles (Memorial) - 37 years (though the park was there for 10 years before)
Braves (Fulton County) - 30 years
Brewers (County Stadium) - 37 years
Mets (Shea) - 44 years
Giants (Candlestick) - 40 years
While most of these were multi-purpose stadiums that were the cool new things when they were built and hated by the '90's, there is no reason to think that the shelf life of the newer stadiums isn't limited either, especially with how technology moves so much faster today.
What is Mind? -- Doesn't Matter!
What is Matter? -- Never Mind!