Why did my chain start coming off on the right side of the chainring sometimes when I am pedaling in high gear? I have a 7 speed bike with one chain ring. Recently the chain has started coming off the right side of the chainring occasionally when I am pedaling. My bike has about 2000 miles on it and is about 7 years old. I ride in the northeast in winter so it is exposed to road salt. 

I think the chain is coming off when I am in 7th gear and the chain is angling in to the chainring from the right side where the smallest sprocket (7th gear) on my rear wheel is. 

I suppose this angle is too steep so that the teeth on the chain ring do not fit into the spaces in the chain but hit the edge of the chain and the chain falls off.

I checked the crank and I don't think it is loose. I can't reproduce the problem deliberately.

I see on the internet some people change their chain at 2000 miles but some wait much longer. Could a worn chain cause the problem?

Is there any other likely explanation?

Why would this start to happen now? 

Is there any way to fix it? 

Thanks
Chains life spans have huge differences, depending on both conditions and maintenance and quality of the chain itself. If you have 2000 miles on the chain and it has been exposed to road salt, it is very likely that it has been worn out. If you are lucky, you can fix it by just replacing the chain, but if the chain is badly worn you may have replace the cassette and at worst case the chainring too. 

If you have to replace the chain ring, there are so-called narrow-wide chainrings that are designed to hold the chain better in single chainring setups.