Quote: "Only the parents of a child have the knowledge on whether they are well-rounded enough to play a particular game."
Exactly true.
Quote: "If I sell your child an 18-rated game, then I'm raising your child how I want. Do you want me to introduce him to heroin and crack cocaine too?"
I don't get that at all.
If I'm the parent of that child and told them not to buy 18+ games and there they are shopping for them, that's my failure in parenting to teach them the value of respecting my wishes. It's all about what values the child has received from their parents. If I'm the shopkeeper, how am I to know if you, the parent are OK with video game human violence or nudity but totally against any sort of depiction harm towards animals and thinks super mario brothers or deer hunter is deplorable.
The shopkeeper only influences the child for a moment but the parent has much, much, much more influence over their decisions. Same applies with the drug dealer. For community members with kids, how does it make you feel, as a parent, when your child makes the
right decision based on the values you've instilled in them. I don't know myself but I imagine it must feel fantastic.
That said, if a store wants a policy not to sell certain rated games or bar entrance altogether to individuals based on age, that's totally their prerogative and the communities in which they operate may indeed be grateful but that should be the store owners decision not the governments.