I dig through ads on Craigslist a lot, just for fun, and I commonly come across snowmobiles that say "Only 3,500 miles!"

How long do these things last before getting too dangerous and unreliable?

mileage dosen't really mean shit. its all in how well maintained the sled was. Ive ridden an aincent sled that ran like new because it was well kept.

mileage dosen't really mean shit. its all in how well maintained the sled was. Ive ridden an aincent sled that ran like new because it was well kept.
Exactly

I dig through ads on Craigslist a lot, just for fun, and I commonly come across snowmobiles that say "Only 3,500 miles!"

How long do these things last before getting too dangerous and unreliable?

3500 miles isn't a huge amount on anything. maybe a child's tricycle but that's about it.