I live in the middle of nowhere on a dirt road, and my kids were allowed on that road as soon as they could power their way up the driveway to get to it - which probably was about age 5. But paved roads? Around here, with curvey roads with no sidewalks or shoulders, 14.
First real road ride my younger son went on by himself at age 14 resulted in a crash when some idiot cut a blind corner, going about 40, and drove him into a ditch.
Needless to say, I prefer they stick to their mountain bikes & run the forest trails.....