Here is an updated, 2005, list of old-timers

ATS-3 - September 9, 2000

From ATSOCC Events Log

Breaker feeding power to the ATS receive antenna,Generator charger and sump pump tripped. ACTS was off last night due to inability to point the receive antennas. I started trouble shooting at 0800Z (4:00am EDT)the problem was found and repaired by 1400Z (10am EDT). Critter had chewed through the power cable where it was exposed for about six inches and water on the exposed cable had caused the short. ATS-3 satellite antennas were repointed, satellite clock reset and both VHF regulators turned on by 1400Z.

ATS-3 - 1997

From NASA SP-4217 "Beyond the Ionosphere' Chapter 6

"The following ATS is the oldest active communications satellite by a wide margin. Launched in November 1967, it is still in service more than 28 years later. Among its widest known achievements are the first full-disk, color Earth images transmitted from a satellite. Its imaging capability has served during disaster situations, from the Mexico earthquake to the Mount St. Helens eruption. ATS-3 experiments included VHF and C-band communications, a color spin-scan camera, an image dissector camera, a mechanically despun antenna, resistojet thrusters, hydrazine propulsion, optical surface experiments, and the measurement of the electron content of the ionosphere and magnetosphere. Because of failures in the hydrogen peroxide systems on ATS-1, ATS-3 was equipped with a hydrazine propulsion system. Its success led to its incorporation on ATS-4 and ATS-5 as the sole propulsion system.

IMP-8 was decommissioned in 2001 after 28 years of operation

From 'Jonathan's Space Report December 5, 1999 :

The oldest spacecraft definitely working are TRW's Pioneer 6, 7 and 8
interplanetary probes; Pioneer 6 is 34 years old.

The oldest Earth satellite is probably the Hughes HS-306 satellite ATS 3, in
geostationary orbit since 1967. However, the 1966 ITT/US Army Secor
geodetic satellite EGRS 7 was reported in 1980 to be still transmitting,
so it's just possible it's still around. NATO IIA, a Ford (now Loral)
satellite from 1970, is in the stable geostationary location at 105W,
and it's not possible to tell whether it is still under control.

Here I give a list of the oldest satellites not definitely known to be defunct.
I'd be very interested in hearing of definite death dates for any of
these, or any active ones I've missed.

Satellite...............Launch.......... Type...........................................Status

EGRS 7 ...............1966 .........ITT Secor US Army ..................Last report 1980
ATS 3 ...................1967 .........Hughes HS-306 NASA ............Still working 1990s
NATO IIA............. 1970......... Ford Skynet NATO/USAF.........status unknown
DSCS II F-1......... 1971......... TRW DSCS 2 USAF................since 1979
IMP 8.................... 1973......... GSFC IMP NASA.................... Still OK 1998
Skynet IIB............ 1974.......... Marconi Skynet UK MoD .........Still OK 1994
LES 8................... 1976........... Lincon Labs USAF................. Still OK 1992
LES 9................... 1976.......... Lincon Labs USAF.................. Still OK 1992
Marisat 103.......... 1976.......... Hughes HS-333 INMARSAT... Active
GOES 2............... 1977.......... Ford SMS NOAA...................... Active
FLTSATCOM 1....1978........... TRW FLTSAT USN................ Active
GOES 3............... 1978........... Ford SMS NOAA..................... Active