The case could be made that not letting the people who will have to deal with the consequences for longer have a larger say is also discrimination. To be clear I'm not advocating for this to be put into practice however I do think that elderly should vote with this point in the back of their minds.

By this line of reasoning you could indeed decide to give less of a voting power to old people, arguing that all political decisions have important long term consequences.

There are plenty of reasons why that would be unacceptable, and I'm sure that's why human right conventions prohibit this kind of discrimination.