A week and a half ago, I was on the highway 100 miles from home with my 1999 Chevy Metro. I had 150lbs of frozen beef (for dog food) in the back of the car.
The engine quit on me in the left lane while doing 70mph. I tried in vain to start it up again in neutral, then switched lanes and pulled over. I called my insurance roadside assistance phone number and got towed off the highway. I rented a U-haul truck with a car dolly trailer, and with the help of a Good Samaritan, got my car up onto the trailer.
I towed it home, threw the beef in the freezer, dropped off the U-haul truck, biked home, and took a look at the car. Had good spark and fuel, and the starter was definitely trying, but it wouldn't start. I took a closer look, and the timing belt had jumped a few teeth. I took a closerer look, and the crankshaft wouldn't turn over all the way. I removed the spark plugs, and one was broken with a 1/2" piece down in the cylinder. I suspect it dropped a valve, and may have cracked a piston.
The car already had lost second gear, was beat up and rusty (rust would probably have killed it within a year or two tops), had a damaged airbag system (air bag light was on), and the engine was low-ish compression before all this. It had never left me stranded before, but it did have a slow oil leak. It also has no air conditioning, and my girlfriend hates it.
That's a good time to get rid of a car.
It was a 1999 with 182,600 miles on it. Our other car is a 1992 with 198,000 miles on it. There's no rule of thumb for age or miles that is really useful for this decision. Get rid of it when you have a reason (a real reason) to suspect its reliability, and when that reason cannot be fixed. Or when it fails catastrophically like mine did.
I've seen people dump a Toyota at 150,000 miles that I thought were insane to sell because it ran perfectly (and the next owner is still driving it), and I've seen someone sell a Saturn at 110,000 miles that made me wonder what took so long (the car looked 100% perfect cosmetically but was total garbage mechanically). It depends on the car.
They're Toyotas. Change the oil, keep up on maintenance, and they're at least 250k mile cars.
Also, this.