Cleopatra VII was about 18 years old when she became queen of Egypt in 51 BC.
Her father, Ptolemy XII Auletes, died by March 51 BC, and Cleopatra inherited the throne under his succession arrangements. She initially ruled jointly with her younger brother Ptolemy XIII, following the Ptolemaic practice of pairing a queen with a male royal co-ruler. Cleopatra was born in 69 BC, so the conventional calculation places her accession at approximately 18.
The word “about” matters because ancient records do not preserve Cleopatra’s exact birthday. Modern biographies generally give her birth as early 69 BC, while some allow late 70 BC; nevertheless, 18 is the standard quiz answer for her accession in 51 BC.
She was not immediately an uncontested sole monarch. A power struggle with Ptolemy XIII forced her temporarily out of Alexandria, and she regained authority with Julius Caesar’s military and political support in 47 BC. Cleopatra’s reign lasted until 30 BC, when Egypt became a Roman province after her defeat by Octavian.