Okay, but the person you replied to doesn't, and instead of providing an actual answer to their question, you posed a false equivalence between proving your age and buying a computer.
>You have to pay a cost to go out in public, since there are nudity laws. You have to pay a cost to use an airport or a train station. You have to pay a fee to prove that you own a car. And so on.
You are purposefully muddying the waters by being lax with your use of language. The "cost" you "pay" by wearing appropriate attire in public is fundamentally different from the actual cost you actually pay when you engage in commerce; one is a trade of freedoms and the other is a trade of goods and/or services. If your argument is that the freedom you have to trade in exchange for the freedom to access the Internet, is that of not having to show an ID, that's one thing. If you also have to add a recurring monetary cost then that's another.
If you don't have an answer to the question of why someone should have to pay again to use the Internet beyond "*shrug* just 'cause, dude. Who cares?", then maybe you shouldn't have said anything.
Of course I have an answer. To do something about the unlimited firehose of porn, violence, divisive, and addictive content that has been pointed at children for the past generation or so.
There's literally nothing confusing about the "why" in this discussion.
The fact that bad people use the "what about the children" argument regularly for bad reasons doesn't mean that all such arguments are bogus.
In fact, it's an indication of exactly the opposite, it's so regularly used because there is a broad consensus that we need to protect children from harm which is why it's often effective as an arguing tool.