A good doctor should be able to figure out an accurate diagnosis for either of those by the time you are in your late teens or early twenties if you are exhibiting symptoms; men tend to develop schizophrenia younger than women. With BP, your symptoms will likely get worse as you age. I don't know if that's true for schizophrenia or not. Just because you are younger than average doesn't mean you don't have BP. My issues started when I was a child and I was not correctly diagnosed until I was in my 20s. The real key is to get the proper diagnosis so that you don't waste too much time being treated for the wrong thing. That can have dire consequences. And, if you aren't happy with a doctor or diagnosis, it is crucial to get a second opinion.
Yes and adult female can have symptoms and not be diagosed, for many BP's exp type II's long periods of domancy are not abnormal - often it takes a major event for diagnosis to occur.
Symptoms normally manifest in early - mid teens and early adulthood for BP - I have no idea about other illnesses. There is a current vogue to diagnose children as young as three as BP but I fall into the camp of not believieng enough research has been done to show this conclusively and its really only a common diagnosis in the US right now.
Any particluar reason why you are asking?