I was young not that many years ago and I can assure you that is far from the case, drugs are freely available for anyone who wants them. It's easier for a teenager to buy weed than alcohol, heck it's easier for them to buy heroin than alcohol, it's cheaper for a young person to buy an xtasy tab for a night out than to pay for alcohol.
Teenagers are rebellious, making something illegal is probably more of an incentive than a barrier lol
Wouldn't it be better if heroin wasn't freely available for anyone who wanted it?