If you are familiar with XML ...
 
Open up Program Files\Stardock Games\Elemental\data\English\ElementalDefs.xml
 
In there, there a bunch of dynasty settings:
- <!-- 
** Dynasty stuff **
  --> // 
 
  <Dynasty_MaxNumChildren>4</Dynasty_MaxNumChildren>
 
  <Dynasty_ChanceToMarry>0.1</Dynasty_ChanceToMarry>
 
  <Dynasty_ChanceToGiveBirth>0.1</Dynasty_ChanceToGiveBirth>
 
  <Dynasty_TurnAgeGrown>100</Dynasty_TurnAgeGrown>
 
  <Dynasty_TurnAgeOld>600</Dynasty_TurnAgeOld>
 
  <Dynasty_MinTurnsBetweenChildren>20</Dynasty_MinTurnsBetweenChildren>
 
  <Dynasty_MinTurnsUntilMarriage>50</Dynasty_MinTurnsUntilMarriage>
 
  <Dynasty_ChanceToChangeAllegiance>0.67</Dynasty_ChanceToChangeAllegiance>
<Dynasty_TurnAgeGrown>XXX</Dynasty_TurnAgeGrown>
Just make XXX to be how many turns you want to wait.
Copy the ENTIRE file, and create a new ElementalDefs.xml in Elemental\Mods (you may need to create that folder). In game options, select "Enable Mods"
This is untested but it should work. (: