Women over the age of 21 are often addressed as “Mama” and men over the age of 35 are often addressed as “Mzee”. Children generally refer to adults as Aunt or Uncle, even if there is not a familial relationship. Once a personal relationship has developed, you may be able to address a person by their title and first name, first name alone, or nickname. Wait for the Kenyan to determine that your friendship has reached this level of intimacy.

Source: http://www.commisceo-global.com/country-guides/kenya-guide