Green’s Dictionary of Slang

1. (orig. and mainly US) an old person, or a more experienced person.

1818		‘A. Burton’ Adventures of Johnny Newcome II 77: An Oldster with a Gunter’s scale Bestow’d his blows as fast as hail.
1831		Navy at Home I 6: The envied post of mate of the watch and oldster in the birth.
1834		Marryat Peter Simple (1911) 50: We all had leave from the first lieutenant to go to Portsdown fair, but he would only allow the oldsters to sleep on shore.
1865		H. Kingsley Hillyars and Burtons (1870) 257: And the answer to this was, among the oldsters, that there was something deuced queer.
1886		Referee 7 Mar. in Ware (1909) 187/2: You mustn’t trust the oldsters too implicitly when they endeavour to persuade you, as they always will, that there never was such a time as their time.
1903		A.H. Lewis Boss 48: Three or four oldsters of the neighborhood, like a council or a little court about a monarch [...] were sitting about him.
1905		Sun. Times (Perth) 19 Mar. 1/1: Her heir and his daughter registered swift time while the oldsters were philandering.
1912		E. Pugh City Of The World 233: These oldsters [...] tell him plainly that he is foolish to kick against the pricks.
1943		M. Constiner ‘The Turkey Buzzard Blues’ in Ruhm Hard-Boiled Detective (1977) 247: The oldster put on a great display of exasperation.
1953		T. Runyon In For Life 78: I asked how much time that gray-haired oldster over there had done.
1960		J.D. Macdonald Slam the Big Door (1961) 172: Come to this retirement paradise, all you senior citizens. (This seems more palatable than ‘oldsters’).
1971		L. Bangs in Psychotic Reactions (1988) 5: Cut the senile drool [...] Oldster!
1981		J. Ellroy Brown’s Requiem 173: Jane came out of the elevator followed by an ascetic looking oldster with a cane.
1990		(con. early 1950s) J. Ellroy L.A. Confidential 11: The oldsters zipped up and walked out.
2000		M. Amis Experience 112: A magazine called Modern Maturity, with the usual couple of great-shape oldsters on its cover.

2. a veteran.

1953

T. Runyon In For Life 186: A good many grizzled oldsters sneered at the whole idea.