I think by the time people are 13 or 14 years old because by then they know what they want to vote. Otherwise the voting age could be by the kids responsibility, have them take some sort of test to see how responsible they are and if they are responsible then they can vote, and they have the choice to vote if they are responsible enough.

The Argument for a Fixed Age of Majority as Opposed to Sociological Test of Maturity

When considering the legal age of majority, there is a debate between the concept of a static age and maturity tests. I think there are very good reasons for choosing a static age. The primary reason is that a definitive time frame is given for planning. The second reason is that a specific age provides protection to minors. The third reason is more an argument against maturity tests.

Regardless of the age decided upon by a culture, having a static age of majority gives a minor, and indirectly her/his guardians, a reliable time-frame within which to plan the transition process from child to adult. Having age milestones in a culture allow us to gauge maturity, intellectual capacity, physical development, sociological adjustment, and many other aspects of growth. This is one of the most difficult transitions in life and having a set point when that transition is complete has proven to be beneficial to individuals and society at large.

A very important aspect of static ages of majority in a society is the protection it brings to minors, especially in relation to adults who would abuse thier power and knowledge to manipulate others less mature or experienced than themselves. In situations where minors are coerced into financial, sexual, or emotional situations they aren't mature enough to handle, a static age of maturity protects them by law and unwritten social contracts. Without these protections we would expose children to harm which is easily avoidable.

The last reason for a static age of maturity actually is in concern of the nature of maturity tests. Any test based on objective facts is an easy to administer and easy to grade test. Any maturity test would not only be subjective, but also decides whether the individual taking it has access to the basic rights of a society. That is a problematic mix. We have disagreements on who would design the test, who would judge results, and what do we do with those who are unable to pass even well into what is currently considered adulthood. Most importantly, it is this question of who decides what determines maturity that concerns me. There is too much potential for maladjusted people to deny rights to others through corruption of the process.

Despite the fact that there may be people who are denied options that they are mature enough to handle and these same options are provided to people who may never be ready to handle them, having a static age of majority has benefits that outweigh this inconvenience. Having a reliable time-frame for planning the transition to adulthood and the protections provided against predators are strong enough reasons to advise patience to those who are mature enough, but not old enough, to be adults. And, if they don't have the patience to wait, maybe they don't have the maturity after all.

@sammwich  "UHDR is not a legally binding document".  Not true, it is binding. Nowadays it is "ius gentium", i.e. it's regarded compulsory international law for and in any State on Earth.

Personally: I would say NO.  The declaration of HUMAN rights should not be a part of a constitution.  Aside from the fact that a Constitution is more of a frame work document that later layers of law and legislation are applied to making it an ill fit for such a complex set of declarations to directly incorporated it could be said that should we decide to travel to an "alien" world in future and discover a form of life, even microbial, that  our fundamental standards would not protect this species from eradication by Humans.