There have been kings as old as 3... they never ruled though uncle's often did that for them and then kicked them out when they became older... But thats not the point here now is it.
Naboo doesn't have a royal family they just choose a dictator for the next four years or so... what kind of planet would vote for a 14 year old? She's 14.... Imagine if she was in a funny mood "And from now on thursday will be pink bunny day, where everybody has to wear pink bunny clothes."