If someone sets up the seat for you and explains how to shift and brake and rides with you a little bit, I'm sure learning would go much faster than if you were just left in a driveway with a bike and told to figure it out.
As for how I learned to ride, my dad took me to a hill, sat me on the bike and pushed me down. I learned how to ride pretty quick...well until I hit a small stump hidden by the grass.
After that I was mostly fine and learned to ride pretty quick.
If you want a kid to learn to ride a bike, remove the fear of failure. Do it on a gently sloping grassy hill that levels out, for example.
When I first learned to ice skate I learned that falling on ice fucking hurts. I didn't learn to ice skate. I learned to not fall over. Then my friend dressed me up in his team's goalie's pads - I pretty much bounced back up again if I fell over. By the end of an hour I could do the sprint exercises with the team and keep up (over short distances anyway!).