I learned to cycle when I was about 6 or 7 years old and have cycled continuously (road and then MTB and back to road) since then.
At the time I learned to cycle I had my arm in a plaster cast to above the elbow joint because I had given myself a greenstick fracture of the radius (leaping off the top bunk and landing badly). I had been to an out patients appointment in the morning and as I recall my mother had decided it wasn't worth sending me to school for the afternoon so she dragged me along to the house of one of her friends.
Well I eventually got bored and went nosing through the friends garage and came across a kids bike in there (no stabilisers). Looking back on it now, Christ knows what I was thinking but I took the bike out onto the cul-de-sac and attempted to ride it.
The first few attempts were pitiful but by the end of the afternoon I had balance, braking and turning figured out and was obviously well pleased with myself.
However, in learning to ride I had worn out the plaster cast across the palm of my hand and had to return to hospital to get it fixed and what really sticks in my mind is the ticking off I got from the doctor/nurse who seemed to be less than impressed with my efforts.