I've suffered from extreme tiredness since 15 & can never seem to wake up. tests revealed no obvious illness or sleep problems. any ideas?

Hard to say: If you have been thoroughly tested and no organic cause could be found, then by definition, you could have chronic fatigue syndrome or it could be something psychological which could be hard to prove. Chronic, smoldering low grade infection is possible but again, hard to prove. Or it could be this is the way you are. Hard to say without knowing what workup had been done or what you look like. Gl.

Narcolepsy?: If "extreme tiredness" means you are excessively sleepy, then you probably have a sleep disorder. Narcolepsy symptoms usually appear in early to mid-adolescence. Narcolepsy is not as rare a most doctors think. If you haven't already done so, see a sleep specialist. If you have, get a second opinion.