See my other quote. They live for many more years then humans.
They dont. Dogmeat the First dies in FO1. In FO2 He comes from the death in a random encounter with other characters who died in FO1.
The one in FO3 could be a descendant, but is more than likely a legacy character.
http://tvtropes.org/pmwiki/pmwiki.php/Main/LegacyCharacter
The only dog to actually outlive a human in Fallout is Rex, who is a special case because he is a cyborg.