The word "user" is ambiguous.
There are two kinds of relationships between an "user" and a computer.
The computer may belong to the employer of the "user" and the "user" receives temporary local access or remote access to it, in order to perform the job's tasks. Or the computer may belong to some company that provides some paid or free services, which involve the local or remote using of a computer.
In such a context, the main purpose of security is indeed to ensure that the "user" cannot use the computer for anything else than what is intended by the computer owner.
The second kind of relationship between a "user" and a computer is when the "user" is supposed to be the owner or the co-owner of the computer. In this case the security should be directed only towards external threats and any security feature which is hidden or which cannot be overridden by the "user" is not acceptable.
Except perhaps in special cases, parental controls should no longer be necessary after a much lower age than usually claimed, as they are useless anyway.
I have grown up in a society where everybody was subjected to parental controls, regardless of age, i.e. regardless whether they were 10 years old, 40 years old or 100 years old.
Among many other things that were taboo, there was no pornography, either in printed form, or in movie theaters or on TV.
Despite this, the young children, at least the young boys, were no more innocent than they would be today given unrestricted access to Internet. At school, after the age of 10 years, whenever there were no adults or girls around, a common pass-time was the telling of various sexual jokes. I have no idea which was the source of those jokes, but there was an enormous number of them and they included pretty much everything that can be seen in a porno movie today. The only difference between the children of that time and those who would be exposed to pornography today was that due to the lack of visual information both those who were telling and those who were listening did not understand many of the words or descriptions included in the jokes.
So even Draconian measures designed to "protect the innocence of the children" fail to achieve their purpose and AFAIK none of those boys who "lost their innocence" by being exposed to pornographic jokes at a low age were influenced in any way by this.