How "old" is a specific snake venom?
According to the following sources, venom resistance has evolved in both snake prey and predators. Also, resistance can be just regional/confined to a certain population. That suggests that arms races occur, where the composition of the venom of a certain species is constantly changing, and so are the defenses of the prey and/or predators who get bitten. In that case, a snake venom is not the same in chemical composition for very long, and might not be the same across populations of the same species of snake. But those differences might be on a smaller scale than you are looking for. I don't know anything about how this affects effectiveness of antivenom. However long it is until the antivenom loses effectiveness, it might be much shorter than the divergence time since that snake species branched off from the evolutionary tree.

Ground squirrels and rattlesnakes: 

"Snakes win: Coevolution of venom function and prey resistance in a rattlesnake-squirrel arms race; 

"Coevolution of venom function and
venom resistance in a rattlesnake predator
and its squirrel prey"

"How is rattlesnake venom like fine wine? Both have regional varieties"

(I wanted to make this a comment but don't have the rep for that).