Because society has a long-standing agreement that a phase of life called "childhood" exists, during which an individual is not fully developed and can't be trusted to make decisions about his or her future. It may be somewhat arbitrarily defined at 18, but I don't see why people who need to be protected against entering into contracts as well as receiving a slew of rights and protections can be expected to have meaningful input into the management of their country. Someone who can't even join the military could vote to send their older fellow countrymen into war. I find the idea that the voting age can be lower than the age of majority very strange, and I have strong doubts that the age of majority should be lowered to 16 - on account of protecting childhood.
> This seems to affect our dramatically in our modern era, as many of our older folks are still around and skewing the generational differences towards conservatism.
Wait, what. You meant to come out against ageism, then you criticize old people for voting "wrong"?
All in all, if you take a step back from you criticisms, there's a common point: Legitimacy. Submitting a ballot every few years is a very weak foundation for a government as sprawling and powerful as most modern and western ones. Fiddling the knobs of who can vote does little more than shift the balance of power inside the political class.