Mobile phones are bad for all ages. Official documents on the health-risks are not going to come to light for some time, just as the tobacco industry could keep all research into smoking quiet for years, so too are telecommunications giants (who are even richer than tobacco companies) keeping any negative information quiet as long as they can.
But without a doubt prolonged usage of such a tiny device as a mobile phone can cause eye-strain and RSI, even if they don't give you brain cancer.
Youngsters are easy targets for advertisers, so there are many negative social (as opposed to medical) effects of mobile phone usage too.
@KennyR: On your opinion of popularity: I could not agree more.
