I actually more concerned about the other person....28mph can cause serious harm & death. Whatever the insurance costs, sorry. Even if it can't be 'affordable'. And required for all ages if that ys what you allow.
Yes I would require insurance. And a basic practical driving test, and a more detailed theory test as is for a full driving licence. And yes I would allow any age to drive one of these provided that they can pass the test.
A maximum speed of 28 MPH certainly could kill, but is less dangerous than faster speeds. Hence the need for a good knowledge of the highway code.
20 MPH is safer still and should be the new norm in built up areas.
A lightweight and speed limited 4 wheel vehicle is safer than two wheels, and probably safer than riding a horse.