Orient Express - Cheap Chinese Rims / Wheels / Forks / Frames / Cranks / Etc

There is a gallon jar of acetone in my grandparents garage, will it have denatured/whatever the term is over the years?

It should be pretty much as manufactured. In a sealed container out of daylight, acetone doesn't have an opportunity to get up to much. It can be photochemically decomposed into acetic acid in the presence of water, light and a catalyst. Worst case your jar might contain a small amount of vinegar, which is unlikely to be a problem for common household uses.