Uthman (may Allah be pleased with him) was martyred at the age of ninety or eighty-eight

It is narrated from Qatadah that when Uthman bin Affan (may Allah be pleased with him) was martyred, he was ninety (90) or eighty-eight (88) years old.

It is narrated from Abu Nu'aym; he said: Uthman bin Affan (may Allah be pleased with him) was martyred on Friday, with twelve days remaining of Dhu al-Hijjah in the year thirty-five (35) AH, and the period of his caliphate was twelve years.

It is narrated from Abu Abdullah, the freed slave (mawla) of Shaddad bin al-Had; he said: I saw Uthman bin Affan (may Allah be pleased with him) on the pulpit on Friday. He was wearing a coarse lower garment (izar) from Aden, worth four or five dirhams, and over him was a sheet made in Kufa, "mumashshaqah" (dyed with red ochre). He was of firm, well-fleshed build (sturdy), his beard was long, and his face was very handsome.

It is narrated from Sayyidah Aishah (may Allah be pleased with her); she said: The first stone for the building of the mosque was carried by the Prophet (peace and blessings be upon him) himself, then by Sayyiduna Abu Bakr (may Allah be pleased with him), then Sayyiduna Umar (may Allah be pleased with him) carried another stone, and then Sayyiduna Uthman (may Allah be pleased with him) carried another stone. So I said: "O Messenger of Allah! Do you not see how these people are helping you?" Then he (peace and blessings be upon him) said: "O Aishah! These are my successors (caliphs) after me."
This hadith is sahih (authentic) on the condition of the two Shaykhs (Bukhari and Muslim), but they did not record it. This report became well known through a weak (wahi) chain of Muhammad bin al-Fadl bin Atiyyah, and for that reason it was abandoned (by the people of knowledge).