You're right! And people still commit theft, murder, and every other crime, so obviously we shouldn't have any laws at all! It makes perfect sense!
Strawman, those crimes are crimes against another person, smoking is not a crime in and of itself because it falls within self determination. Also, there is no age restriction on crimes against another, because by their very nature those acts take away or restrict the rights of others, which is why it is illegal.
I can understand why you think the way you do, people who don't understand the implications of the abuse of the legislature can't grasp the idea of restricting laws to those necessary to protect people's rights, without venturing into the government attempting to protect people from themselves, which is unconstitutional and unethical.
What's the difference? Marijuana is less harmful than cigarettes.
Also, it should be 18+ for everything. You're an adult, you can go and fight and die for your country yet you can't purchase alcohol and other vices?
Not true at all, Marijuana burns hotter, and has far more carcinogenic chemicals, and also causes mental and physical impairment in the short term, which limits the ability to operate machinery (drive) and perform complex tasks requiring full control of motor functions.