Having the child sit upfront next to the driver as soon as they are old enough to be allowed to do so probably helps a lot to "mentally prepare" them for driving when the time is right. Biking on neighborhood roads to build some traffic awareness as well.
A full year before he reached driving age, I started asking my DS to not be distracted while in the car (no staring at his phone) and pay attention to the road while I am driving, to everything I do in traffic, and mentally make his own decisions (and compare with mine), to "simulate" an in-traffic experience. This is what a driving instructor - in the notoriously chaotic city in a developing country I had grown up in - had asked me to do when I was getting ready to drive and I really benefited from that. We had very limited time with an instructor, that too on a clunky stick shift, and the hours of make-believe driving sitting next to various family members I felt had prepared me well to not panic and be confident (except once when the stick shift stalled badly in busy traffic up a slope

).
It helped that my DS really wanted to drive, and took to it quite well when he got his learner's permit.
Some people are just not ready to drive at age 16 for whatever reason. I don't think they should be forced, as one semi-bad incident can affect their future ability to drive. I know of many who weren't ready for a while, and eventually started driving in their early-20s and became fine drivers. Maybe their confidence had improved, or concentration, or hand-eye coordination, executive skills, whatever. People develop and change at different rates, and that's fine. And some may never drive as well, which also should be fine. Some of them even manage to learn at an older age if and when they absolutely need to.