I sold my first one with 88,000 miles and it was running great, I sold my 2nd one with 94,000 miles, and it was running great. I had a 3 month old one that quit running at 12,000 miles. (I drove it into a farm truck at highway speed and it was totalled). The one I have now only has 52,000 on it. I adjusted the valves at 6000, 12,000, and 24,000. I pulled the valve cover at 48,000 and they were all still in spec. No magic juice, just regular oil and filter changes with good automotive/diesel rated oil. I always run a couple ounces of marvel mystery oil in a tank of gas. No rust in the tank, no corrosion in the carbs. If cared for, they last a long time.
I never had petcock issues. As part of maintenance I look for any leakage or stains. I also move the lever through the positions at least onece per tank. if I even THINK something is wrong, I buy a new factory unit and replace it. I have never replace two on the same bike. $70 is cheap insurance over that many miles.
An older bike with low mileage might have old tires on it. I would really inspect the tires before I took off for long or "spirited" rides.