Apart from the issue of personal inconvenience, there is a safety issue.
It would be possible to get, eventually a full bike licence, and never have driven/ridden anything with a steering wheel and get straight into a Morgan three wheeler and drive off. I'm well aware a three wheeler without a reverse gear could be driven at 16 on a bike licence in the dim and distant, but traffic is heavier now. You cannot now drive a Morgan on 'L' plates either.
Had there been a real safety issue with the driving of a three wheeler at a young age, the insurance cost would have been high. It wasn't, and owners of vintage Morgans had no problem in arranging cover for their offspring at 17.