Tests would be a pretty bad idea. It is easy to imagine the ways that someone could use that to attack their political opponents. Similar things were used to disenfranchise voters in the past. Also, it is too easy to corrupt the legitimacy of such a test. All a person would need to do is get a heads up of how the test works and practice for it. Or, have the test designed to be too easy to pass. It’s easy to say “make it impartial, scientific, and dignified”, but that doesn’t mean it will be. I seriously doubt any governmental body ever has or will be that trustworthy. An actual age limit would be objective and clear though, making it much more practical.