Abu al-Zinba' Rawh bin al-Faraj narrates from Yahya bin Bukayr: "Zayd bin Thabit (may Allah be pleased with him) died in the year 45 AH, in the fifty-sixth year of his life. Some people say that he died in the year 48 or 59 AH, because the Messenger of Allah (peace and blessings be upon him) permitted him to take part in the fighting on the day of the Battle of the Trench when he was fifteen years old, and the Trench took place in Shawwal of the year 4 AH. There is disagreement about his death."
Hadith Referenceالمعجم الكبير للطبراني / باب الزاي / 4736