Ohh so many...
Well sadly 15 is not an unusual age to start, and in that sense a 15 year old is the tobacco companies dream. A teenage mind is different both emotionally and chemically from an adults and nicotine
that much more addictive.
If a 15 y/o begins to smoke they are much more likely to continue it through life and, of course, the exposure to the cigarettes and all the toxins goes on that much longer.