Its clearly the opposite of a mature fail. Although I'm not entirely sure what that is either..
It is one that is complete, cultivated and cultured. One that has developed and is fit, full-blown, full-fledged and full-grown - maybe even fully grown and in full bloom. It certainly is in its prime, matured and mellowed. It has come of age and been perfected, prepared. It is prime and ready because it is seasoned, settled and sophisticated.