The Constitution never explicitly ensures the right to vote, as it does the right to speech, for example. It does require that Representatives be chosen and Senators be elected by "the People," and who comprises "the People" has been expanded by the aforementioned amendments several times. Aside from these requirements, though, the qualifications for voters are left to the states. And as long as the qualifications do not conflict with anything in the Constitution, that right can be withheld. For example, in Texas, persons declared mentally incompetent and felons currently in prison or on probation are denied the right to vote. It is interesting to note that though the 26th Amendment requires that 18-year-olds must be able to vote, states can allow persons younger than 18 to vote, if they chose to.
Also the founding fathers did not view slaves as people. They were talking about rich white land owners when they spoke of people. Now before everyone goes on a IHATE is racist prick lynching sessions I DO NOT BELIEVE THAT IS HUMANE. My grandfather was black, my mothers side is native american and my wife is asian. I do not feel color or gender should ever come into a discussion of human rights.
I do feel though that it is insane that the 18-25 year old demographic (which a large portion of is brainwashed by their profs or just too young to know which way is truly up) can swing a vote one way or another. I also think it is insane that people who have never had to balance a check book in any scheme have equal ssay as those of the country who have and do.