As political scientists Michael X. Delli Carpini and Scott Keeter noted in their 1996 book, “What Americans Don’t Know About Politics and Why It Matters,” political knowledge is not evenly spread among all groups. Membership in some demographic groups correlates with high levels of political knowledge, depending on region, income and education, while other groups tend to correlate with political ignorance.
So, this is the catch: If you wanted to exclude 16- and 17-year-olds on the grounds that they are more likely to be ignorant or misinformed, you would also in effect be arguing against other demographics having a say.
Of course, you could object that the issue here is not so much about knowledge, but the capacity for political judgment. Yet nothing special happens at age 18 — there is no sudden increase in cognitive ability, political wisdom or political knowledge at that age. In fact, in the 1970s, psychologists discovered that the biggest increases in the capacity to reason about politics occur around age 12 when puberty hits. By 16, most people have about as stable an ideology and capacity to reason about politics as they are going to get. Any improvements after that come slowly, if at all.