Very good questions that I’m also curious about myself. Like why not get some mice, let them age, and then do the tests? Because it takes too much time? Seems like drugs like this take several years to develop/test so that wouldn’t make sense.

Because they’re more prone to developing complicating conditions (unrelated to “aging”) that could disrupt what they are testing for? Gives them a more consistent baseline/control group. But even then they can easily get enough mice to eliminate those anomalous mice as they present themselves, and have enough remaining for the experiment.

As to the cancer point, I’d imagine that just by purely numbers they would inherently be at a higher risk of developing cancer relative to an unmedicated person their age.