Even 21 feet may be pushing the envelope - I'm 62 and stay fit so I can cover that distance in 1 second (was a time when I could do close to 30 feet in that time from a dead stop, and a tad over 30 feet after actually moving, but not anymore).
Like Leaning Right said, situational awareness is rule #1. Like sun Tzu said, know yourself and know your enemy. That is what situational awareness is at its most basic level. Example: if I’m walking across a dark empty parking lot and I observe individuals approaching me. I am going to become responsive well before they get within 21 feet. I’m going to prepare to defend myself. The shirt or coat concealing my firearm is going to be moved out of the way. My hand is going on the firearm. and I’m going to maneuver to disrupt their plan of attack, to keep them more than 21 feet away from me, and to force them to reveal their ill intent outside of 21 feet. The situation would be much different in broad daylight with witnesses all over the place.