Would 16 likely be too early for these manifestations to appear? what symptoms would appear as the disease progressed?

Symptoms would include possible paranoia, scattered thought processes, purposeless habits (might be seen as nervous habits, but they are not). there are more but this person should see a professional for diagnosis. Giving you these symptoms may lead you to a false diagnosis, to which no one other than a professional should make. Schizophrenia is a physical disorder as well as a mental one. His/her doctor should be able to pick up on signs if there is a problem. maybe the best help you could give is to pass this person a cold. maybe I should not assume you are referring to an actual person and maybe want to learn about it, but that is not what I thought you were asking.

16 is about the right age when these symptoms start to appear. one of the things to look to is person talking to themselves, is an obvious one, but also avoiding situations is one that isn't so obvious. Like avoiding school because of a threat that isn't there. a friend of mine, her son went through this. He didn't want to go to school, he was afraid of people following him, he would hide at home and look through the windows. He actually suffered from paranoia schizophrenia. He is doing fine now. Takes his meds, lives on his own and works.

The best people to ask are those that you know & spend time with.
They would likely notice subtle personality/behavioral changes that you'd display if you were suffering from a mental illness.

I hope it isn't excessive use of Yahoo! Answers.