Does the rate of slowing depend on your strength in the Power?
Yes, basically; you age at 1/X your normal rate, with X being dependent up to a point on Power strength. The Power acts as a natural rejuvenator on you; even at the point of death you won’t look or feel worse than a normal 65-year-old.
(RJ had to hem and haw a good bit to avoid revealing that slowing != the Ageless Look—there were several spoiler people in the audience, including one who was working on Book Six at the moment.)
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