Having recently been given the privilege of becoming a reviewer for The Catholic Company, I read Saint Thomas Aquinas by Raissa Maritain, a biography translated into English by Julie Kernan. Beginning with Saint Thomas Aquinas's childhood, education and religious formation, the book follows his path up to and beyond his death, discussing also process of his canonization, highlighting some of his major works of literature, the Office of the Blessed Sacrament, and the Summa Theologica.
While I enjoyed the book I'm not sure what the age target was: the vocabulary is a little difficult but I wonder if a teenager would find the tone a little condescending. One note of caution to parents: while most concepts are well explained or defined, the tradition of bodily relics (namely, in which two shrines his arms may be found) is unexpectedly raised without much context. This may be a new and uncomfortable concept to some children. Overall the book is a nice overview of St. Thomas's life and work, and perhaps would work well as story to be read with your child, or a jumping off point for a confirmation student wanting to research Saint Thomas.