There is no contradiction between the idea that poetry has no rules, can never be learned, and the idea that it takes a lifetime to learn anything about poetry. If there were rules it wouldn't take a lifetime.

A lifetime might be Keats', or Rimbaud's, or Spicer's, or O"Hara's.

Or it could be Sophocles'.

Some old people can't write any more. They might have thought they already learned the rules.

Some young people can't write because they don't know enough yet.

"The lyf so short, the craft so long to lerne." Is that the first English language reference to poetry as a "craft"?

Illusory metaphor.