The Alligatorinae-Caimaninae split is actually one of the best-constrained divergence dates for vertebrates. The two clades diverged from one another somewhere between 71 and 64 million years ago (Ma). This was either shortly before, during, or just after the Cretaceous-Paleogene (K-Pg) boundary, which is currently placed at 66 Ma. This point in geologic time is best known as when the non-avian dinosaurs went extinct.
This means that alligatorines have been around for the entire (or almost the entire) Cenozoic, which takes up the leftmost column in the picture above. If they diverged from caimans shortly before the K-Pg boundary, they would have also been present in the Maastrichtian, which is the last age in the Mesozoic.