If you browse YouTube from iPhone safari browser you can view YouTube as Mobile version,On bottom of the page if you click view as desktop view then you are in trouble,you cannot switch back to mobile view because there is no link to do so..
After some research,I have found this problem caused by safari cookies
You have to delete your cookies
How to delete cookies in safari
1,Go to your iPhone home screen
Now visit YouTube again,Now You can see YouTube in mobile version