After some frantic googling I have come back with some initial discoveries:
I looked up a list of players who played over the age of 40 and found that Jerry Rice was the only pass catcher to do it, so we already had his most ancient TD, so then I went to the QB's and started poking around there.
At first I thought George Blanda was gonna make an appearance since he played til he was like 48, problem was he played mostly as a kicker, and only threw TDs on trick plays. His last td pass in the NFL was his oldest with him at 47 throwing to a 26 year old Cliff Branch combining for 73.
Brett Favre was the obvious next choice and he threw a respectable TD pass to an age 33 Randy Moss for a combination of 74 years in his last season (2010).
Next I looked up Warren Moon's last few seasons from 1997-2000. In 2000 he threw one TD at age 44 to the 30 year old Troy Dayton putting him at 74. He didn't throw a TD pass in 1999 and his oldest td's in 97 and 98 were 71 and 72 respectively so just like Blanda his last TD pass in the NFL was his oldest, and while he eclipsed Blanda's mark, he couldn't best Rice and Gannon.
I checked on Tom Brady, and his oldest TD is actually last year to the 31 year old Danny Amendola, putting him at a combined age of 70. If the pair connect this year they'll be the oldest active connection in the NFL at 72.
So then that left the Journeyman QB's Steve DeBerg and Vinny Testaverde. Looking up DeBerg first I see that he didn't play since he was 39 until he was 44 and setting the record as the oldest starting QB in NFL history playing for the Atlanta Falcons in 1998 at the age of 44. Like some of those before him his last TD Pass in the nfl was his oldest throwing to the 33 year old Tony Martin against the Miami Dolphins for a ridiculous combined 77 years of age.
At this point I was CERTAIN that was gonna be the oldest one none of the others even eclipsed 75 let alone Gannon and Rice at 76, so the 77 was gonna be rock solid despite not looking at Testaverde's career. But then I saw he threw a single touchdown for the Patriots in 2006 when he was their back up and he threw it to Troy fucking Brown. With Testaverde at 43 years old and Troy Brown at 35 that was a combined 78 years old! BUT WAIT THERES MORE, I look at Testaverde's last season with 2007 Carolina Panthers to see if there was anyone older. And on November 18th, 2007, a 44 years and 5 day old Vinny threw a 2 yard TD Pass to the 36 years and 57 day old Christian Fauria for a combined Geezer TD Age of 80 years 2 months and 3 days. Incredible.
No other QB over the age of 40 combined for a TD remotely close so I think that's gotta be it. Testaverde to Fauria is the oldest TD in NFL history.
