"What is the minimum age?"
Translation:Qual é a idade mínima?
If you use o que you ask about the meaning of the expression "a idade mínima". If you translate "qual" then it means what the minimum age is.
I was always cofused about the usage of "o que" vs. "qual". So is it everytime like this or are there other situations/meanings to keep in mind, if I have to decide weather to use "o que" or "qual"?