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'.
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.