Nobody knows, because nobody living at the same time ever mentioned her age at a given date.
Some people have said she was "older" than Mohammed. This is very unlikely, because Mohammed was born in 570 and Sawda died in 674. So even if she was only one day older, she would have lived to be 104 years old! This is possible, but we would need better evidence to believe it for certain.
It is true, however, that Mohammed considered divorcing her because she was "old". This probably means that she was older than his other wives. If she was ten years younger than Mohammed, and lived to be 94, she was still 13 years older than the next-oldest wife.
Sawda bint Zamʿa died in 674.
Nobody knows. The Arab historians usually give each person's age at death, together with the year of death. They say that Sawda died in 674 but they don't give her age.
In the same way as anyone else - by declaring in front of witnesses: "There is no god but Allah, and Muhammad is his messenger." There are no records about the specific circumstances surrounding Sawda's conversion, or about her reasons for choosing Islam. However, it occurred in the first six years of Islam, making her a very early convert.
The exact date is not known, but it was sometime between August 613 and August 614._______________________________________________________The 2nd wife was Sawda bint Zama: Sawda bint Zamʿa ibn Qayyis ibn ʿAbd Shams was a wife of the Islamic prophet Muhammad, and considered by Muslims to be a Mother of the Believers.Marriage with Muhammad between 0619 and June 8th, 0632 (13 years)died in 0674 (1339 years ago)Muhammad married her in the month of Shawwal, in the tenth year of His Prophet-hood, a few days after the death of Khadijah. Prior to that, she was married to a paternal cousin of hers called As-Sakran bin ‘Amr. She was considered homely and was older than Muhammad.It is not known precisely when she was born.
Francis Zama was born in 1956.
Huzaima bint Nasser was born in 1884.
Safiyya bint Huyayy was born in 601.
Aliya bint Ali was born in 1911.
Bint al-Huda was born in 1938.
Hafsa bint Umar was born in 606.
Sukayna bint Husayn was born in 676.
Musbah bint Nasser was born in 1884.