WHO Panel: 'Limited Evidence' Suggests Cell Phones 'Possibly Carcinogenic'

What I don't get is this: With more than 5 Billion people using mobile phones, according to the article, wouldn't we have already seen the first waves of a tumor epidemic?

I understand that children are particularly susceptible and the trend in children getting phones is a recent one. But mobile phones have been available en masse for over 20 years -- (I've had one for 15 years) -- so you would think that adults who have had phones for most of that time would have been exposed enough for there to be some obvious effect.