Mismatch

You may argue, based on the previous example of a benefit twenty years later, it is logical to ask only those who are potentially affected (i.e. those below 60-year of age) to vote.

But, I'm sure you see it plainly that's quite impossible, for obvious reasons:

1. Most importantly, the group most benefited (that is, those below the age of 20), is often quite incapable of making a logical and appropriate judgement.
2. It would be seen as a discrimination of age - a taboo that we don't want to get near to.
3. In real life, it is often difficult to tell how long it needs to wait for the benefit.
4. And, in reality, we do not vote for policies one by one (and we don't want to anyway - otherwise we would not need a representative in the council), we vote for persons, or, a choice of different bundles of policies.

Specific for point #1, you may now come to realize it is an inherent problem of democracy: There is often a mismatch between those eligible to vote and those who actually get the benefit, and the temptation is more than real for the former group to choose a policy (or, a candidate or political party) than would benefit themselves rather than the entire society. To put it simply, they balloon up their own purse by taking money from those who do not vote, or vote for the opposition.