I agree that such ideas are not enforceable. We can't just assume all new drivers are 18. You can be any age and pass your driving test.
The logical way forward is within the 12 months of learning that drivers are taught how to drive on the motorway, at night and to understand the dangers of drinking and driving and/or drugging and driving. This makes a lot more sense and would prepare new drivers for the many journeys ahead, instead of wasting Police time on identifying which drivers should or shouldn't be driving past a certain time!