This view of Merlin is canonical with the original written translation of the Arthurian cycle, ["Le Morte d'Arthur". In it, Merlin is said to "remember what is in our future", and to "have no knowledge of what is in our past".
He physically does not age, and this is never explained, only mentioned. There is never any reference to his perception of speech and motion being backwards as well.
However, others have expressed doubt about whether this feature of Merlin can really be found in Malory's Le Morte d'Arthur.
Does Malory's version of Merlin display any kind of "living backwards"? Either in his memories or in anything physical? If so, where exactly is this stated?