Universal suffrage is the fallacy which converts democratic process into mob rule.
Voting is a leadership function. I would not allow a 16 year old to pick the CEO of Microsoft or Exxon, so I CERTAINLY would not allow a 16 year old to pick the President of the United States, arguably the most important position on earth.
Likewise, I would not allow anyone who is not responsible enough to support himself to choose these positions, or anyone who is an habitual drug abuser, or who practices a religion dedicated to evil (e.g. Satanism), or convicted felons, or those on medication for mental illness.
A Republic does not require universal suffrage, our Founders did not (with rare exception) endorse it, and no state in the original union practised universal suffrage. This notion sprang from the social contract theories of Rousseau and the French Revolution, the ideas that ultimately developed into socialism and Marxism.
I let my sons to discuss the stock I own for them. When I get board directors information on directors, I talk to my sons about what is going on and let them read the report and talk with me about what directors they should vote for. I vote for them, since it is in my name, but it is theirs stock.