A very common trope in fiction is the concept of some powerful, forbidden spell/martial art/etc., which is said to have a cost paid in the user's lifespan. Now, oftentimes this literally physically ages the user, but not all the time. When Tien uses this sort of life-force-draining ability in the anime Dragonball Z, for example, over-using it just causes him to collapse and die, still as young as he was before he used it, just dead.
Now, this can obviously be attributed to the magic system of these settings having "life force" as a physical, finite resource that you will eventually run out of, but suppose your setting doesn't allow for that, whether because you're more grounded in realism, or that just isn't compatible with the magic system of your setting. That would mean that this ability would be doing something to the body that will cause you to die younger and younger the more you use it, until eventually over-use will kill you outright.
If the setting's mechanics of life and death were completely grounded in realism, what could this ability be doing to the body that would reliably lower the age you could realistically live to while doing as little to your overall health as possible before it reaches that stage?