Well, analyze the adjective in this case.
old (ōld) pronunciation
adj., old·er, old·est.
1.
1. Having lived or existed for a relatively long time; far advanced in years or life.
2. Relatively advanced in age: Pamela is our oldest child.
2. Made long ago; in existence for many years: an old book.
3. Of or relating to a long life or to people who have had long lives: a ripe old age.
4. Having or exhibiting the physical characteristics of age: a prematurely old face.
5. Having or exhibiting the wisdom of age; mature: a child who is old for his years.
6. Having lived or existed for a specified length of time: She was 12 years old.
7.
1. Belonging to a remote or former period in history; ancient: old fossils.
2. Belonging to or being of an earlier time: her old classmates.
8. often Old Being the earlier or earliest of two or more related objects, stages, versions, or periods.
9. Geology.
1. Having become slower in flow and less vigorous in action. Used of a river.
2. Having become simpler in form and of lower relief. Used of a landform.
10. Exhibiting the effects of time or long use; worn: an old coat.
11. Known through long acquaintance; long familiar: an old friend.
12. Skilled or able through long experience; practiced.
13. often ol' (ōl)
1. Used as an intensive: Come back any old time. Don't give me any ol' excuse.
2. Used to express affection or familiarity: Good ol' Sam.
As I have pointed out above in bold, old refers to, in this case, a "Belonging to a remote or former period in history; ancient: old fossils."
Hence it would imply that these Sith belonged to a former period in history.
However, if this answer does not satisfy you, then please click on the below link.
http://www.google.ca/
Proceed by typing in ancient Sith.