Original: 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.
Simplified: 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.