My reasoning for implementing universal rules for drugs (ie. being 18 or 21 or whatever for everything) is:
1. It's a restriction of one's freedom of choice about whether or not to use drugs to institute a blanket ban. Even if suicide is bad for people, or alcoholism, we let people do it as long as they harm nobody else.
2. It would lead to cleaner, safer supply of drugs. Without the impurities and poor conditions in which drugs are currently taken there would be far fewer deaths.
3. It would be easier to treat addicts, because they could be prescribed clean drugs and slowly weaned off them. Also, there's more incentive to seek help because there are no legal difficulties, and society would view addicts more sympathetically eventually.
4. The criminal thrill of drugs is removed. Kids won't find it so cool if their parents can do it as well.

5. Drug addicts are victims as much as criminals. They're trapped in a cycle of addiction and they need support to get out - like a smoker needs support if they want to quit.