Walt and Michael left the island some time late in November, 2004.
Walt arrived in New York within days of leaving the island and has been raised by his grandmother ever since.
The Oceanic Six are rescued in January, 2005, and report there were NO other survivors.
So, what identity has Walt assumed since leaving the island? He can no longer be Walt Lloyd as this would contradict the testimony of Jack and the Oceanic Six. Is he also lying?