12,000 miles per year is the average, so anything over that is above average. One would expect a 10 year old car to have 120,000 miles on it. If it is your own car the mileage is much less important than the maintenance. But if you are buying a car, the mileage is important because you can't be certain about the maintenance. If you are selling a car, you have to be aware that the buyer is going to put a lot of weight on the mileage. I bought a two year old car once with only 116 miles on it. The owner was disabled in a medical accident the week after buying it and held onto it for two years before selling it. My kid brother did a bunch of yard work for an elderly lady years ago and she gave him the car that was up on blocks in her garage. It was a 52(?) Chevy with less than 2,000 miles on it, her son had put it in the garage when he was sent to Korea and he died there, and she never took it out. Sadly, seems like good car deals often come out of the misfortune of others.

A Yugo with 1 mile on it is a high mileage vehicle. Depends on the vehicle and use/abuse, not the year.

We have a 99 Chevy Venture. I get to drive it while the wife gets to drive the newer car. sigh

We bought it used with I think about 34k on it. I've almost put 100k on it and it still runs like a charm. I do regular maintenance, not just the scheduled oil changes. You just gotta drive smart, you know, everything we are NOT inclined to. But it's survived not just around town trips but road trips from New Mexico to Indiana, New Mexico to California (3 or 4 of those), 2 trips to Las Vegas and I can't really complain. Other than driving a freakin' minivan. double sigh

I own old Toyotas and just sold my Mercedes diesel. Life span for well built, maintained cars used to be 1/2 million miles or more. Newer cars, mostly on the body & interior (unless garaged- I'm in Texas), are crap.
To answer the question directly, high miles on newer cars is 100000 miles.
YMMV

For me, it's when the longest warranty I could buy with the car is expiring. Sometimes this is 100,000 miles and sometimes 120,000. I don't worry too much about major repairs before then, because they are all covered.

I've had to Cherokees that have hit 200k miles and just kept on kicking without any problems aside from minor leaks and a water pump or two. For a well taken care of Cherokee, that's just getting to the point it might be considered high mileage.

Our current family car is a Chrysler Pacifica. It's got less than 90k miles on it and it's on it's 3rd transmission and has had most of the suspension replaced and has had each of the 5 motor mounts replaced at least once and...

I did all of the work on both of the Jeeps with inexpensive parts. I can't do any of the work on the Pacifica aside from oil changes.

High mileage is all relative.

Depends on the car and the condition. I paid $800 bucks for a 94 Lincoln Town car in okay condition with 200,000 miles. It's been 6 months, it still runs great, and I expect this engine, if not the whole car, to make it to 275,000 at least before it is rebuilt. Quality gets you to quantity when it comes to milage.