People think it's very very important that everyone is given the right to vote (I don't but anyway). That's why we let everyone vote. Or do we? We don't let people younger than 18 vote, ever.
Of course that's because we don't really think everyone has the right to vote, just everyone past a certain age. But age is an odd limitation. Why is the number of years you've lived on the earth matter to what rights you get? Aha, because age is actually just a proxy for level of development or maturity.
Now we get to my point; if giving people their rights is so important, isn't the age limitation rather high? We would want to err on the side of giving people too many rights than too few. Presumably there are a lot of 16 and 17 year olds who can't vote and are every bit as mature as 18 or 19 year olds who can. Aren't we denying them their rights.
Go back a hundred years before black people could vote. If someone said, "I'd love for black people to vote, but they tend to be less mature and so we can't." And also suppose that it's a social scientific fact that black people at that time were less mature. Would that be a good argument? Only if the immature ones outnumbered the mature ones 100 to 1 would we even consider denying the one his rights to prevent 100 immature people from voting.
But I don't see that happening with age. I don't personally have a conflict because I don't determine people's rights by my own personal feelings. But for those who look inward and decide, "yes, all people should have the right to vote," holding to that consistently should make them change some other beliefs.