That he was a child is immaterial to the immature and radical nature of his idea. It is backstory, colour, to describe the stereotypical nature of the idea. That a child creates an idea doesn't mean that it is bad per se, but should give adults pause before signing up to it themselves. That the young are always more extreme than the old should be common knowledge.

I think that you just disagree with the idea, which is fine, but you might consider attacking it on its merits from the perspective of the people who agree with it, not have a meta-discussion about when it arose in his mind, how "immature" it is, etc.

Norquist, the Republicans who sign the pledge, and the voters who demand it all apparently think that it's a good idea, it's been effective from their perspective, and it's ideologically consistent, so having someone who doesn't like its outcome call it "extreme" and "immature" doesn't really mean anything. It's just another flavor of ad hominem attack.