The difference between legal and illegal is simple; if something is legal it is permitted under law, if it is illegal, it is not. The line between legal and decriminalized is not so clear-cut. To understand the difference we look to the Merriam-Webster Dictionary definition of each of the three words.
conforming to or permitted by law or established rules
not according to or authorized by law
to remove or reduce the criminal classification or status of; especially : to repeal a strict ban on while keeping under some form of regulation
According to these definitions, if something is legal it is permitted with few or no restrictions, if it is illegal it is not permitted, and if it is decriminalized it is permitted with tight regulations and/or restrictions that must be obeyed or it will be viewed as illegal.
Why is it OK for my parents to smoke and drink but not for me, isnt that hypocritical?
This is a simple question to answer. It is OK for your parents to use alcohol and tobacco because the law says it is OK. If the law ever changed to allow teens to legally use these substances than it would technically be OK for them as well. This is currently NOT the case. For this reason, and this reason alone, your parents are not acting hypocritically when they tell you not to drink alcohol or smoke cigarettes even if they do these things themselves. What may be a bit hypocritical is them telling you not to do these things for health reasons when they do them, but this is likely a matter of parents hoping you will learn from their mistakes more than a matter of a parental do as I say, not as I do attitude.
It is important that teens learn to respect the law and the system we have for changing laws that are unjust or wrong. A healthy respect for the law and the legislative system is a lesson that should not be brushed aside with a my parents do it so why cant I dismissive attitude. If you do not like the laws, work to change them, but ignoring them just so you can get high is a sign of the very immaturity that caused legislatures to enact age limits on alcohol and tobacco use to begin with.
When a law is truly unfair, when it does more harm than good, working to change it is noble but in the case of alcohol, tobacco and even drug use there is little noble in the argument I like to get high because it feels neato and it isnt fair to limit my freedom to do so. Drug and substance control laws are not your typical discriminatory laws that need to be changed and therefore, should be respected.
Alcohol, tobacco and street drugs cost millions every year in healthcare costs, criminal costs and social costs. As our awareness of the real harm of these substances grows the personal freedom argument becomes weaker and weaker. For now, it is more important that you respect the laws on alcohol and tobacco rather than obsess over how unfair it is that parents can drink and smoke while teens cannot. It is a waste of your energy to make an issue of laws that stop you from harming yourself, and potentially harming others. The law feels that teens lack the maturity to handle these harmful substances responsibly and ignoring those laws only validates that stand.