A very good point, which highlights an inconsistency in the law. Could have been easily avoided if the age was set at 16/17, not 18. That should have been picked up at committee stage.
There's a similar inconsistency in our approach to sex - old enough to do it at 16, but not to watch it until 18!
If it has even a mild deterrent effect, it will have done
some good. And since it only stops people doing something they shouldn't have done in the first place, I don't see it doing any harm. Unfortunately, it will be impossible to quantify the benefit unless they've secretly fitted air pollution detectors into smokers' cars.