You'd be surprised.
Privates out of basic training are driving them. To even get to a position where you are the highest ranking members on your crew means you are smart enough to stay out of trouble and do your job effectively.
I'm not a tank commander or anything, but there might be some kind of test you have to pass in order to become a tank commander. In an anti-armor Infantry company, you usually have to be able to identify enough vehicles to know you're not destroying a friendly.