When is old "old enough"

[BlackFireN2O]

When is the actual time for an engine to absolutely need a rebuild. Is there a certain amount of miles that an engine can accumulate until the whole powertrain is the prime example of the old adage "you can't teach an old dog new tricks?" Can a stocck motor take some of the bolt-on's that most mustang owners throw at their cars? Is converting a 100,000+ mile car to a carb and porting the heads and adding some nitrous just too much for the factory blue oval, or will it take it? All opinions are greatly appreciated, Thanks!