Is there a logical (in a fantasy world) reason for wizards to grow more powerful as they age? Is it merely because they train, or do they change to get more power?
closed as primarily opinion-based by Mołot, Hohmannfan, TrEs-2b, Vincent, Thucydides Sep 5 '16 at 0:31
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Well you have different options.
1) Wizards train more and get better with there powers. This happens in real life, look at a 25 year old versus a 6 year old, in virtually any discipline the 25 year old will win. This plateaus off in real life due to negative effects of aging countering the gains through practise. For wizards age is less of an issue so they continue to gain power.
2) If magic can be done through amulets or talismans older wizards have had time to accumulate more of these so they are more powerful.
3) If magic is cast from inner energy older wizards might have more inner energy due to them losing there physical energy.
For most myths, the apparent age of a powerful wizard is a trope, intended to illustrate that the wizard is wise and exchanged their interest in bodily power for the more potent powers of the mind. It can often also communicate a lack of interest in bodily pleasures as well. Use of the trope means that age and power aren't necessarily correlated, but age instead serves as a sort of visual shorthand concerning the wizard's character. See Tolkien's Gandalf for instance. Created to appear aged, Gandalf allowed the trope to operate both in- and out-of-story.
Less often, age and power are correlated, but the causality could fun either way. Power might be the result of a lifetime of study, or extremes of power may have allowed the wizard to age prematurely or beyond their normal lifespan.