The obvious place to start, is with Aisha. Muhammad married her when he was approximately age 55 and she was approximately age 6. Muhammad had sex with her at age 9. She was 18-19 when Muhammad died. I feel compelled to explain my use of the term “approximate.” The Hadith is not consistent on this. How could it be? It takes legitimate inspiration for multiple versions of events to be recorded without flaw. The Hadith in its various forms is anything but flawless. In this case, it only varies by a year either way.
"'A'isha (Allah be pleased with her) reported that Allah's Apostle (may peace be upon him) married her when she was seven years old, and he was taken to his house as a bride when she was nine, and her dolls were with her; and when he (the Holy Prophet) died she was eighteen years old." [Sahih Muslim Book 008, Number 3311]
Aisha also reported these events herself:
"The Messenger of God married me in Shawwal in the tenth year after of his prophethood, three years before the Migration as I was six years old. I was nine years old when he consummated the marriage with me. [Tabaqat, 8:58; Abu Dawud, Nikah 34; Ansab al-ashraf, 1:410; Usd al-ghaba, 5:501] "Narrated Hisham's father: Khadija died three years before the Prophet departed to Medina. He stayed in Medina for two years or so and then he married 'Aisha. At this time she was a girl of six years of age, and he consummated that marriage when she was nine years old." [Sahih Bukhari Volume 5, Book 58, Number 236:]