Green’s Dictionary of Slang

(orig. US) an old man, usu. a disreputable one.

1949		A.I. Bezzerides Thieves’ Market 138: Even that old cocker with the rooster-look stiffening his face at sight of the girl.
1976		R. Price Blood Brothers 152: ‘You believe this old cocker,’ Butler said to Stony.
1983		R. Price Breaks 241: I was [...] gettin’ into that stupid riff with them old cockers.
1989		H. Rawson Dict. of Invective (1991) 276: old cocker (or gaffer or fart). An old man, especially a silly or senile one.