Effects of Repeated Spellcasting on Aging

Use of magic, particularly the casting of spells, is generally believed to be a direct way to focused application of power to great advantage with few or no side effects.
This, is, in most important respects, entirely in error.
Spell casting is a particularly difficult facet of using magic, and summoning, rituals, and object contained effects are all much simpler. While it is direct, it is hardly focused, and side effects are often immediately evident to anyone who sees unperfected spell casting performed.
But more importantly, the long-term side effects on the caster, specifically their body, can turn out to be a serious disadvantage in the long term.
It has been observed that incidence of chronic illnesses, including pain, fatigue, dementia, and disorientation, is much higher among those who practiced spell casting for many years. This correlation appears more pronounced the longer the individual practiced the casting of spells.
Obviously, it has not gone unimagined by those who have actually observed this correlation that there might actually be some causation involved. Namely, that spell casting damages the body.
The dominant theory points to conservation of mass and energy as the root cause. Clearly the casting of a spell is an expenditure of power, but where does that come from? Advocates of this theory put forth that the energy expended is actually converted mass from the caster's muscles, bones, and nervous systems.
If true, this certainly explains the aching pains that afflicted older spell casters, and goes a long way to explaining mental effects such as disorientation. The primary counter, however, is that these chronic effects are not universal. There have been aged spell casters who never experienced these problems. And, also, that the amount of body converted to energy would have to be so minimal to have avoided detection that it couldn't possibly account for the energy expended by spell casting.
The counter-counter argument begins to talk about the feeding patterns of demons, a priori self-evident quality and magnitude of the power of the soul, and so on.