Can someone enlighten me the rationale behind the fact that mentally disabled citizens are allowed to vote while a 17 year old doesn't have that right, no matter how smart or knowledgeable you are?
The rationale is that the age limit is easy to define.
We could debate why is age 18 the age we decided upon but that's a different topic. At least that's a criteria we can apply equally to everyone.
There is no specific definition for 'mentally disabled'. Plus, just because someone may fall into that category doesn't mean they don't deserve equal opportunity at representation.
Bottom line, the age restriction is something we can apply equally to everyone. The 'mentally disabled' label is not.
The existing answer explains the age restriction, so let me focus on the first part of the question.
Voting rights for mentally disabled persons are different in different countries.
In United States, National Alliance on Mental Illness (NAMI) has a Web site that contains some key information on voting rights for mentally disabled.
U.S. Federal laws protects the right to vote for every person with a mental disability who understands what it means to vote.
About 30 states in US have laws in their constitutions that can limit people with mental disabilities from voting if they have been ruled "mentally incapacitated," or incompetent, by a court. This means they have been determined unable to manage their own affairs or make specific life decisions, which, other than voting for candidates, can include managing their money, entering a contract, making medical decisions or caring for their children. — The Atlantic
As for the reasons why voting rights are often limited for these people:
- A person who has been ruled by the court of incapable taking decisions, may become a target of fraud. Simply speaking, they can be manipulated by someone to vote for a candidate who is a preference for the malefactor, not the voter.
- Manipulation can be associated with other crimes, like violence or bribery, which is especially violent against a defenseless person.
- This can come to a large scale: consider a hospice, for example.
- Medicines used for treatment mental illnesses may produce hallucinations or other kinds of mental disorder. Even sleeping pills can!
In any case, in democratic countries everyone has right to vote. Even someone whose voting right been limited, may appeal — probably, with some legal help from organizations like NAMI.
Yet another thing is that mentally disabled persons usually struggle for survival and don't care much about their voting right. :-(
Young people are easy to manipulate and find it difficult to compromise. As the Pope Benedict XVI said political positions that promotes a very black-and-white picture of human society and rules out any compromise have disastrous results. Young people tend to hold these black-white political positions.
For example the genocide in Cambodia was orchestrated by young people that came back from studies in Europe. They were killing anyone who dared to complain about the social changes they were enforcing. Another example would be Red Guards in China. It was a fanatic student mass, patrolling the streets and attacking conservatives. In Germany NSDAP was supported by the young people. The average age of those joining between 1925 and 1932 was slightly under twenty-nine. Personally I would rather advocate for rising the minimum age when you are eligible for voting to 30 or maybe even 35.
The term mentally disabled can be overused. It would be extremely difficult to agree on the definitions. In Soviet Union conservatives that believed in God and opposed progressive regime were diagnosed with Sluggish Schizophrenia and deprived of public rights. In United States the political opponents of Donald Trump were accusing him of mental disorder.
Striping mentally disabled out of voting rights open the doors for systematic abuse. The number of mentally disabled people that vote is negligible, but limiting their public rights could have dire consequence in the future, once the precedent is established.