Chapter 1 - Section 1.1 - Functions from the Numerical, Algebraic, and Graphical Viewpoints - Exercises - Page 55: 51e

By the age of $14$ months, $ 90\%$ or more children are speaking in at least single words.

Using the table from part c, the first for which p(t) is over 90, is t=14, $\mathrm{p}(14)=91.1.$ So, by the age of 14 months, $ 90\%$ or more children are speaking in at least single words.