We can only speculate at what the death certificate means - I too imagined that "general decay" would usually refer to old people.
The only conclusion we can draw from it is that she died of natural causes rather than an accident or suicide.
Have you looked for her burial? If you're lucky, there may be an annotation in the margin giving a cause of death. Also it's worth looking at the newspaper archives.
You say there's no disability mentioned on the census, but does it say she goes to school? You may be able to find school registers for her. It wouldn't explain her death, but may rule out a long term disability.
