Of course, the first one is which animals? Normal lab rats and mice are not perfect, but there are the best models and most used.
For rats and mice and studying longevity, the first challenge is to have enough 24 months old animals. They are more expensive, but studying aging on young animals breads of genetically transformed animals is more “dangerous” for the reliability of the results.
Even if you give one billion dollars to the best scientist in the world, he will be unable to let these rats or mice live without a new therapy. It means the relevance of findings of significant results of maximal lifespan would be quite important (very very difficult to falsify).
Remark: study the fertility of female rats or mice could be even more interesting because faster. Female rats cease to be fertile when they are about 12 months old.