Complex sentence: His mother, Catherine of Valois, was then 20 years old and, as Charles VI's daughter, was viewed with considerable suspicion by English nobles and prevented from having a full role in her son's upbringing.
Split into simpler sentences: His mother, Catherine of Valois, was then 20 years old. As Charles VI's daughter, she was viewed with considerable suspicion by English nobles and prevented from playing a full role in her son's upbringing.