Sayyed Mohammad Al-Musawi, Sayyed Mohammad al-Musawi is originally from Iraq and heads up the World Ahlul Bayt Islamic League in London. Other than being involved in various humanitarian projects, he frequently responds to... Answered 4 months ago

The most authentic sources indicate that Fatima al-Zahra (SA) was born five years after the Bi'thah (Prophethood declaration  ) which is three years after the Esraa'.
Al-Kafi;1:458

and Kashf al-Ghummah by al-Erbali; 1:449,

and al-Manaaqib by Ibn Shahr Ashoob 3:357.

Authentic evidence from Abi Jafar (AS) stated that her birth was five years after Bi'thah. Al-Kafi 1:457 and from Abi Abdullah Al-Sadiq (AS) ; Dala'il al-Imamah 18/79 and from Imam Al-Redha (AS) ; Tareesh Al-A'immah by Ibn Abi al-Thalj.

Some Sunni books narrated different narrations but not authentic as the statements of Ahlul Bayt (AS) mentioned above.

'The age of Fatima Al-Zahra (AS) when she was martyred was eighteen years according to the most authentic sources.

Wassalam.