Q15. "When the 26th amendment was being considered, why did the Congress settle on 18 as the minimum age? Why not 17 or 16?"
A. To be sure, you'd have to check the Congressional Record to read the debates. I'm not sure they are on-line, especially for that period of time, but if not, then any library should have the Record.
I suspect the most likely reason for picking 18 is that, at that time, in the early 70's with the Vietnam War in full gear, it was the opinion of the Congress that anyone old enough to be sent off to kill and be killed in a war for the U.S. is also old enough to vote on the people who would make such policies.
As noted in Question 14 above, the Constitution is merely setting a threshold, a bottom limit in age over which all persons must be allowed to vote. If a state wishes to allow the vote for 16- or 17-year olds, it has the ability to do so, for any election, local, state, or national.