I'm A Banana
Potassium-Rich
Given that all forms of power in D&D (including skill ranks and wealth) are tied to level, it is just unfeasible that a low-level character would wield real power (not mere figurehead power) when high level characters exist.
I'd disagree that it's unfeasable. I would say that it probably doesn't happen a lot, but that it's still entirely possible for all the reasons PS posted. Loyalty and obedience come from more than just the ability of the one demanding it to punch you in the face. They could respect him because his bloodline is all aasimar, obeying him because he is obviously touched by the gods (kind of a "power by rarity of type"). They could obey him because he can command troops or champions that are high-level, and the only reason he does that is because he's the one who controlls the massive golems guarding his treasure valut that he uses to pay his champions or troops (more economic power). They could obey him because he is RELATED to powerful champions or troops (power through association), or because a prophecy spoke about his bookmark (power through fantasy magic), or because he's the only one who the dragon will deal with (power through the random chance of danger).
Why would a D&D world, with its massive variation in personal power levels and its daytime-soap-opera frequency of problems, manage to have rulers who were actually fit to rule when we've rarely managed it in thousands of years of history in the relatively tame real world?
Because there are ways of getting 20th level paladins to obey you when you're first level, and, sometimes, if you're 23rd level, the 20th level paladins won't obey you.