Laws that can’t be enforced should not be passed.  It is impossible to tell someone’s age simply by looking at them.  Many 17-year-olds look a lot older and many 18 or older teens look a lot younger than 18.  It also is impossible to determine if someone in a passing vehicle is under 18, so how will the law be enforced.  Will police start “age profiling” to find offenders?   It would be nice to know the rules.