Oldest still in orbit, non-operational:
Vanguard 1 (1958)
Oldest operational in any orbit:
Pioneer 6 (heliocentric, launched Dec. 16, 1965); I was not able to see any contacts after 2000, but that one was clearly successful)
Oldest operational in Earth orbit seems to be
AO-7 , launched in 1974
There is an older one,
ATS-3, but I couldn't confirm ontacts after 2000. Launch was in 1967.