Locke, Sawyer and Juliet hold "Jones" and Cunningham prisoner

Locke, Sawyer and Juliet hold Charles "Jones" Widmore and Cunningham hostage while Mattingly's body lies close by. Consider Widmore's name isn't actually Jones, we can assume that Cunningham, Jones and Mattingly were actually members of the US military team that the others killed. Widmore and the others just took their uniforms. Widmore and Cunningham briefly speak to each other in Latin which lets Juliet know that they are others because she also speaks Latin.

How old are the original natives if they speak Latin and have kept the dead language alive all these years?