Heroin is just opium, just like morphine, oxycodone, etc.
Methamphetamine is sold as adderal, and closely related forms as ritalin, etc.
Grocerie stores already sell many many things with no age restriction that can kill you in seconds. Half the cleaning aisle for a start.
The right answer is that they would still need to be controlled just like cigarettes and alcohol.
Nannies should care about babies, not older children.
I’m against the general “what about the kids” rhetoric, but in this case it’s both important and easy to implement, because sellers already card you for the aforementioned purchases.
Edit: Btw, I did pick up smoking when I was 14. Abandoned it half a year later. The development of kids to adults needs exploration. If you steal that, you steal childhood. And, as history from the seventies to nowadays proves, if you begin to regulate something that might seem necessary, there will rise assholes that regulate way too much. Find another way to minimize harm.