Simplify the following text for a Grade 9 native English-speaking person.
Original: Their eldest son , Ranuccio was not yet old enough to rule the Duchy alone so his mother , Margherita acted as regent until her son was old enough to govern alone .
Simplified: Their eldest son , Ranuccio was not yet old enough to rule the Duchy alone so Margherita acted as regent until her son was old enough to govern alone .