You've asked questions that I'd group into two categories, and that both deserve substantial answers. if you don't mind, I'll answer the second one first, and the other when I find a moment.
As to societally staggering our acceptance of risks... I still say that those "staggerings" are largely arbitrary. How, exactly, is swimming in shark infested waters (which one can do with no regulation or adult present) any less dangerous than using a nipple clamp (which one needs to be 18 to even buy, as minors aren't allowed into sex shops despite ages of consent to sex being lower... see what I mean about arbitrary and contrary to logic)? Please, please do explain that one...
I'm not sure where you live, but where I live, beaches are closed if there's a particular hazard and swimming is prohibited. If a parent knowingly let their child swim in shark infested waters, then they could be prosecuted for child endangerment.
That said, not all risks are made illegal because there is a general reluctance to criminalize children as an attempt to manage their risks, because criminalizing them creates a negative effect by itself. So there really isn't an effective way to stop children from engaging in some dangerous acts without criminalizing them.
What we tend to be able to do though, is to minimize risks by putting limits on how adults interact with them. For instance, in the US, it is not illegal for underaged persons to drink alcohol (in most states) There is a comparatively small fine for them if they buy alcohol, or are found in public possession, but the weight of the law comes down on those over 21 who would buy alcohol for minors, or bars who would serve them.
In your example, for instance, I don't know your exact jurisdiction, but I'd wager that the law has a relatively low, or non-existent penalty for minors entering a sex shop, and a fairly heavy one for sex shops allowing minors in or making sales to them.
I can't say I agree with the prohibition on minors in sex shops, especially those who are above the age of sexual consent. But that's a "risk" that certain populations have decided to act on.
But I'm sure you see the general basis behind the policies, that acting to minimize risks to minors, it is fairly effective to regulate adults, who can be expected to understand and attempt to minimize risks to themselves, and it would be fairly ineffective and likely counterproductive to try to directly regulate the actions of minors. "You're under arrest for your own good" is only useful in very extreme cases.
