Shortly after the marriage , the oldest of Neeltje 's daughters had a child of her own , and so at only 23 years old Dorus became known as `` Opa '' in Den Helder where he lived . Shortly after the marriage , the oldest of Neeltje 's daughters had a child of her own , and so at only 23 years old Dorus became known as Opa in Den Helder where he lived .