First off- why would anyone really care if they don't have long left to live in comparison to younger people? Whilst most will do right, there are the few who won't care/may make a careless vote purely because they don't care. If every vote counts, doesn't that mean that one old person who isn't far from the grave could perhaps be the difference between a good and bad outcome? - The answer is yes. Furthermore, if they are going to pass soon, why should they have a say? After all, shouldn't it be what they younger people want as they are the future generations rather than the old people who won't live to be a part of what they voted for, perhaps putting the wishes of the younger people at wish?
By comparison, young people are clueless when it comes to politics. Over time, they see the impact of various policies for better or worse. That you can't just increase taxes to fund every social program. That for every social program comes those who abuse them. Another factor is, the older you are, the more you have invested into our government. Unfortunately, we gave the entitled generation to deal with today. A generation that believes they deserve everything handed to them from birth. Not just the necessities that your parents are to provide till your of age, not just the rights you inherit when you become of age but they somehow think that the government should pay for their basic needs (home, food, health insurance etc) after they become adults. Sure, then they could just work a part time job for extras but if people work less if at all, were is the tax money to pay for it going to come from? Let me guess, tax the rich and industry, right? So lets get rid of more incentive to earn money and push more industry over seas making even fewer places to fund your easy life.
Because the elders have experience and wisdom, they have the capability to vote for what they have seen work and vote against what they've experienced negatively. This works towards a better community and a stronger future for the younger generations. As a well-known fact/quote says, with age comes wisdom, and if wisdom is what leads to a better community/world/country, shouldn't the vote of the wise be counted for as much as one of a young person?
Politicians don't just make decisions that affect young people, or issues regarding future events. For example, retirement and medical policies are always going to be more impactful on the elderly before anyone else. They should absolutely have a vote as well. Besides, how would you even implement such a condition? What age would you limit voting? People die very young, while others may live well into their 100's.