You think it's axiomatic, but I think we need to prove it.

In America it's taken as axiomatic that only children can learn languages.

This axiom is unquestionably true.

It may be true, but it's hardly axiomatic.

Cole's Axiom: The sum of the intelligence on the planet is a constant; the population is growing.

Poe's Law is an axiom suggesting that it's difficult, if not impossible, to distinguish between parodies of religious or other fundamentalism and its genuine proponents, since they both seem equally insane.