Her age was lowered in that report so that no questions would be raised about her virginity, and by extension, the legitimacy of Muhammad's offspring through her.
Source: "All of these specific references to the bride's age reinforce Aisha's pre-menarcheal status and, implicitly, her virginity. They also suggest the variability of Aisha's age in the historical record." - D. A. Spellberg, Politics, Gender, and the Islamic Past.
In reality, she had been betrothed to Jubair Ibn al Mut'am, a pagan, before her engagement to Muhammad. Jubair eventually annulled their engagement. In at least two separate sources it is pointed out that Asma bint Abi Bakr, Aisha's half-sister, was 100 years old when she died in 73 AH. That would make Aisha 90 in 73 AH.
Source: "Asma died in 73 A.H. at the age of one hundred years. She was ten years older than her sister Aisha." - Ibn Kathir, Al-bidayya wal-nihaya
"She was the sister of Aisha Siddiqa, wife of the Holy Prophet, and was ten years older than her. … In 73 A.H. … Asma died at the age of one hundred years." - Mishkat al-Masabih