The article describes a bad programmer as one whose programs “die young”. I would guess that Knuth is saying is that the longest one of his programs lived (was used?) for 12 years?

If that is what he meant, I presume this remark was written well in the past, as TeX has lasted way more than 12 years.

That makes sense. His cover letter is dated 1974, and TeX was released 1978.