Mileage and size is the thing.
Let's say its a 90 VN750 with 50K miles, thats not too bad,
or let's say a 88 Goldwing 1100 with 167K miles, that's alot.
But consider the goldwing probably went across country and long trips, while the 750 was probably used as a town cruiser and weekend warrior.
The guys i know that work at LMTRIKE, where they do trike conversions, said they had an old 80s something goldwing with over 200K miles on it and still ran great.
And they've seen smaller bikes with less miles run like crap.
But like wib714 said "a lot depends on the maintenance the bike really recieved!"
My gf's bro, bought a 84 honda shadow 700 with 28K miles on it, my 01 vn750 has 19K. He asked if 28K was alot, i told him, no not really.
Sometimes its better to have an older bike with high miles, cuz hey you know it runs and hasn't been sitting for years and years. If you get an older bike with low low miles, i hope it was taken care of and not just sitting and sitting.
Also it depends on your area. The riding season here in Minnesota is way shorter than that of Florida. I do good to put on 6 to 8 K a year here in MN, while people living in Florida, can ride and put on way way more.
Also, its a big difference, if the bike has been sitting/stored out in the HEAT and RAIN, or the COLD and the SNOW.
So all in all, it's not easy to determine what is high miles. The smaller the motor, or the larger the motor, makes a difference.
I wouldn't want to buy a 1990's 400cc bike that has 50K miles on it, but I would buy a 90's 1100 with 50K on it.
Hope that helps.
I would say assume the average rider about 6K a year, so if the bike is a 1990, it may have 126,000 miles. Wow!! That's 21 years, and 6K miles a year!
Now 6K a year is an average, it may be high or low, depending on where you live.
My 2001 vn750 has 19K on it, I bought it with 6k on it about 3 years ago.
So my bike is now 10yrs old, so you would think, assuming 6k a year, it would have 60K miles on it, not 19K.
My first bike was a 78 Suzuki gs750, bought it with 14K miles on it back in 2006. Paid $300 for it, fixed it up, rode it.
So it was 28 years old. So assuming 6k a year is 168K miles!! So it had really low miles. I sold it 2 years later with 21K on it for $1K.
Maybe my average of 6K is way off??!! Last year I put on 5.5K, and guys I ride with say they can easily do 9K a year!! So like I said, 6K is an average.
Hope this helps some