What is considered "High Mileage" now days?

High mileage = more miles than I want on a particular car I'm looking to buy. 

My 135i is 5 years old with 27k miles. I wouldn't have considered it with more than about 40k.

I think age, more than miles, is what starts to cause any given car to develop nuisance issues. As long as the drivetrain has had somewhat appropriate fluids/filters maintenance they can keep running essentially forever provided no hard parts actually break. Example: my 1988 Mazda 323 1.6i had 423,000 miles and 15 years on its original engine and 5-speed. It had gone through 1 clutch, 2 cv axles, 2 front wheel bearings, 1 rear bearing, 1 radiator, 1 spark igniter (weird early Mazda EFI thing), 2 timing belt/water pump sets, hose sets, 1 motor mount set, and 3 sets of struts, and a couple of alternators. All were changed out either on schedule or before any catastrophic failure occurred. As for the engine itself, I never had to remove a single component, ever. That drivetrain made every bit as much power at 423K miles at is did at 23K.               BUT,                 A whole crap ton of little things had just gotten old and either stopped working or broke from just plain age. The drivetrain may as well have been brand new, but the rest of the car was just dead of old age.