What is one requirement to vote in federal elections?

To vote in federal elections, one critical requirement is that individuals must be at least 18 years old. This age limit was established by the 26th Amendment to the United States Constitution, ratified in 1971, which specifically prohibits the denial of the right to vote to citizens aged 18 years and older based on age alone. This amendment was introduced largely in response to arguments that if individuals were old enough to be drafted for military service, they should also have the right to vote.

The other requirements mentioned, such as being a registered Democrat, being at least 21 years old, or being a property owner, are not universal stipulations for voting in federal elections. Voting eligibility primarily centers around age, citizenship, residency, and registration without regard to political affiliation or property ownership. Thus, the correct answer is firmly rooted in the legal standards set for voting rights in the United States.