Never! You don't even need a lot of training if you're willing to "bring it." Here's a fun game to try with a friend: Clear out some space in a room. Hardwood floor, carpet, doesn't matter. Lie down on your back. Have your friend start by standing in front of you, by your feet. His or her job is to run around you and kick you in the head. Your job is to spin on your back and keep your feet in kicking position always pointed at your friend.
So does that mean we are sort of ... kind of ... maybe agreeing that people that aren't pretty well trained are screwed when being attacked while on the ground?