The brothel was discovered in a two-storey building in the nearby Monti district of Rome, Il Messaggero reported.
Prosecutors say it operated for a number of years before being discovered in 2012, with women's services reportedly being bought for €30.
During a routine check in the street, a customer was caught walking into the brothel hand-in-hand with a “young foreigner wearing ostentatious and revealing clothes,” the newspaper said.
After further investigation, the authorities discovered a well-organized brothel, with the 90-year-old handling the money, taking appointments and supplying condoms to the girls.
The woman, named as Marisa M and originally from Bologna, was arrested for the first time in the 1990s for also running a brothel in the same area.
Aged 69 at the time, she was managing the place with two other women.
By Anna Pujol Mazzini