i had an iPhone 6 and my health app worked perfectly fine, switced to iPhone 6s and the health app did not work. my recent fix is documented below
turn off "icloud" and "find my iPhone"
plug phone to itunes and back up (if you purchased apps, make sure you click on transfer purchased apps)
once back up up is complete you are going to restore iPhone to factory settings
once reset is complete go through with the set up(Set up as a new phone)
dont worry about setting up Siri and other items. its not important to do so yet
at this point you should be able to click on the health app. Go to the setting where you can add your age and gender, input that information
leave that hat screen and then go back in. You should see the information previously documented.
at this point you can plug your iPhone to your computer and open itunes
restore back up and you should have all of your old information and your health app should work.
this process should take about an hour or so to complete