Downvoted posts do not get automatically reviewed or deleted just for having a low score. After an answer reaches -3 (lower on Meta sites), it is grayed out, but this only affects appearance. It remains a first-class answer that may be voted on, accepted, flagged, edited, cited, filed, stamped, or numbered.
Our criteria for deletion do not have functionality to automatically delete individual answers solely on the basis of score and/or score plus the passage of sufficient time. The Roomba scripts (see the link above) do provide for automated deletion of a question, and a low scoring answer can be one of the criteria used, but never the only one. A question will never be deleted just because it has a low-scoring answer - the question itself must also meet criteria for auto-deletion.