It's only a recommendation, not a strict instruction. It's the age group that the author is targeting and is assumed to have the requisite background knowledge and reasoning skills required to follow the material. It's understood to vary between individuals.

This is similar to 'intended audience' section of technical books for adults. Do you find those insulting?

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eimrine 11 hours ago | parent [-]

He requested that age requirement, it makes him looking like an etatist. His family will never raise Terence Tao even if their boy will be exactly same qualities.

Some kids use to solder earlier than at 10y.o. so the intended audience is wrong. If the kid has not learned what is a transistor by the age of 5 yet, they are not an intended audience.

	▲	goku12 3 hours ago \| parent [-]
		> He requested that age requirement, Who? > it makes him looking like an etatist. Pardon me, but what does that mean? Is that a typo, or am I simply ignorant? > Some kids use to solder earlier than at 10y.o. so the intended audience is wrong. If the kid has not learned what is a transistor by the age of 5 yet, they are not an intended audience. I don't know. I can't speak for anyone else, but I do have a history related to that. No one around me knew anything about transistors or soldering. I learned those myself by referring books from the library and few others that I convinced my parents to buy me. But it was well after I was 5. The learning was rapid once I got started. That's why I confidently said that the age is a guideline and that it may vary between individuals. Learning in children is quite chaotic and it's difficult to establish hard and fast rules.