I think I started learning manual transmissions when I was about 8 years old, riding with my father. He'd pick less busy stretches of road and tell me I had to shift for him, while he operated the clutch. At first, he told me what gear to move to next, then started to just say "shift" and expect me to have figured out we need to upshift or downshift, and eventually he would just warn me I was on duty and then expect me to anticipate and synchronize with his pedal actions without further voice signals.
When I was tall enough, he'd have me move his car around on our long driveway so I learned to feather the clutch for start/stop maneuvers in first gear. By the time I was getting my learner's permit and hitting the road for the first time, I already had the theory and practice for the drive-train and could focus on the more important traffic rules and vigilance.
I still remember his "graduation" test for me, which was taking me on a ride to a nearby very steep hill and parallel parking. Then, he gave me the keys and said very sincerely that I needed to pull out, drive around the neighborhood, return, and park back in the same spot without burning the clutch and without stalling the engine...