Details of the narration: Mus'ab bin Abd Allah al-Zubayri mentioned it in "Nasab Quraysh" (p. 26), the apparent meaning of which is that he (Ibn 'Abbas) was thirteen years old; however, Imam al-Bayhaqi narrated in "Ma'rifat al-Sunan wa-l-Athar" (1309) via the route of Isma'il bin Ishaq al-Qadi that he said: I heard Mus'ab bin Abd Allah al-Zubayri say: "On the day the Messenger of Allah (peace and blessings be upon him) passed away, Abd Allah bin 'Abbas was fourteen years old". Imam al-Shafi'i also stated this view with certainty.

Reference / Source: Furthermore, al-Bayhaqi reported from al-Waqidi the view of thirteen years, and through Abu al-'Aliyah from Ibn 'Abbas (may Allah be pleased with them both) the age of twelve years is reported. The detailed discussion of all these views can be seen in "Fath al-Bari" (15/ 168).