I'm asking this question not for fear of dangerous powder breakdowns but I have some 10 year old stuff still unused and don't want to load with it if the burn rate is changed from age. I reload for accuracy and fun and don't want to waste my time.

I"ve heard aslong as the packaging has never been openned it usually has a shelf life of about 15 to 20 years don't mark my word I could be wrong. I have a 8 lbs keg of green dot I got from a friend the stuff is about 7 years old and was never openned then I openned it about 1 year ago, so the powder was 6 years old when I openned it, Loads fine in my .38 specials and shotshells. Usually you can tell if powder is going bad by the smell, I would check a SAAMI bulletin it will give you more detail about how to tell if powder is bad.

Depends on the type of powder. Alot of people use pulldown surplus powder that might be 30 or 40 years old and get good results, most of this is IMR or ball powders. Some double base powders tend to leach out nitroglycerine over time and become unstable and shouldn't be left sitting around for decades.

I just used up a unopened 8lb jug of powder from the late 70s or early 80s .It was just as good as new .I used a load from that time . IMO if the powder jug was sealed and not subjected to high temp . it should be fine .Look at it dose it look ok/same as other of the same kind ? Smell it if it smells same as other it should be ok. If it smells strange use it as ferdelizer or burn it .

All depends on how you stored it. if it was kept in an air-tight contaner in a cool dry environment, it can last virtually forever. I have fired .45 ACP ball (FA 18) from WWI which was still giving 820 FPS....