Kimani Murage, who has 30 grandchildren, said he hoped to become a veterinarian.
His classmates' average age is seven years old, and two of his grandchildren attend more advanced classes in the same school in Kenya's Eldoret district, about 270km northwest of the capital Nairobi.
Headmistress Jane Obinchu told the Daily Nation the school first thought Murage's interest was a joke.
"So we told him to come this year in first term, then we could see about it. It was a total surprise when he showed up - in school uniform!" she was quoted as saying.
Mau Mau movement
Murage said he was part of the Mau Mau movement that fought against British colonial rule in Kenya in the 1950s and wanted to learn to read so he could count his hoped-for compensation.
"When this new government came to power and promised to pay Mau Mau veterans compensation, I decided to try to get education since it was free... when they start paying us, I do not want any young man trying to rob me due to illiteracy," he told the East African Standard newspaper.
"I do not want any young man trying to rob me due to illiteracy"
Primary school pupil, aged 84
Kenya registered the Mau Mau movement in November, which lawyers said could help surviving fighters obtain compensation.
President Mwai Kibaki's National Rainbow Coalition (NARC), which trounced long-time leader Daniel arap Moi's Kenya African National Union (KANU) in elections in December 2002, introduced a free primary education scheme in the east African nation in January last year.