Voting requires you to understand politics, and they've rightly or wrongly decided you need to be 18.
I don't see how the two things are related to be honest.
Entering an engineering degree requires you to have a good understanding of higher maths (a level at least), or significant experience in the field. While some rare 14 year olds do grasp it pretty well, it takes most of us until we're 17/18 to grasp it properly, and some can enter direct from industry after apprenticeship.
I don't see politics being much different. You might, at 16, understand that 1+1=2, but can you do calculus, politically speaking? As there are, as far as I know, no GCSEs in politics (could be wrong!), I would assume it best to take the apprenticeship route, and have some experience before being let loose with the designers pen/mouse. I don't see how you get the experience required without a period of dealing with adult situations and expectations.