1. One who is not just old, but is so old that he/she has basically make a name for themselves by being old; They have made being old seem cool.
2. A slang, rude nickname for one that is old, but does not make it cool; In this case, their oldness is the cause of their downfall.
1. That oldster is 113 years old.
2. That oldster gave me a 15 page essay due tomorrow.