In general dogs do not get more tired due to age. They become more tired due to undiagnosed illness. This is assuming the dog's environment, and diet have remained constant.

You should have a full blood panel run. Besides checking for the usual things such as CBCs, blood sugar, also make sure serum calcium, potassium/sodium, and lactic acid levels are checked. A liver profile should be included. If this comes back all negative then consider running tests for thyroid hormone levels.