You can have ED just as soon as you have erections. I wouldn't worry too much about it while you're still pre-pubescent but at 19 it's certainly a legitimate problem. Smoking, excercise and diet are things that usually cause ED on a long term basis. Stress can be more immediate but there are dozens of other possible causes.
I'd talk to your doctor about this. At 19 you have every reason to expect that you can regain healthy function.