As a parent, I find the age recommendations insulting.

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?

	▲	eimrine 8 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.