I spend a year studying for my computer science degree from the University of California at Santa Barbara at the University College of Wales in Aberystwith. It was one of the best years of my life, and, in addition to leaning a lot about the subject, I learned about it from a non-US-centric point of view, which was quite interesting. I also learned a lot about the local culture (Aberystwith had a non-university population of about 15,000 and 41 pubs) and made some life-long friends. If an opportunity to study abroad lands in your lap, you'd be a fool not to grab it with both hands and squeeze all you can out of it. You will be learning computer science for the rest of your career, but you don't get many chances to live in a student community in another country.

I'll keep this short. Firstly, "basic reasoning skills" as a bar is, as you seem to agree, arbitrary and difficult to define. Age, on the other hand, is objective and is based on our legal traditions going back centuries. It is also the basis of a legal definition of adulthood implying enforceabilty of contracts, legal liability, etc. While I agree there are a lot of 18-year-olds who don't have much judgement, they can at least put thier lives on the line in defense of the country in the military, and that carries some weight. To get back to basic reasoning skills, I can easily make an argument that the most erudite, educated, accomplished person who does not agree with me does not posess "basic reasoning skills". I might, were I in power, disqualify you. And you might disqualify me. What you seem to be arguing for is the establishment of something along the lines of, if not Plato's philosopher kings, at least political participation by those who some authority has deemed up to the task. The problem I have is that I wouldn't trust anyone to make that selection. You shouldn't either.