Daenerys refers to Missandei often in the "A Song of Ice and Fire" book series as her "little scribe," because the girl is so small. When she's introduced in the show during season three, she appears to be much older. Her age is unconfirmed in the show, but you can take your own guesses based on her picture here. Also the slavers of Astapor offer her with no prompting in the book "A Storm of Swords." In the show during season three, Daenerys demands they give her Missandei (as a gift).