I am a 59 year old US citizen currently in Shanghai on a local plus job & applied for work permit.

My current contract is for three years with a multinational company and I submitted my application for three years but I only got work permit for one year.
Does the China government have strict guidelines around age and retirement or do I need to renew the work permit annually. Can you please explain to me why I have only one year work permit. How do I get an extension.