Text always uses either regular or medium weight; it doesn’t use light or bold, because light and bold weights don’t read well at small sizes.
Text isn’t smaller than 11 points, even when the user chooses the extra-small text size.
…t size is key to the readability of your text. If the user can’t read it, the design does not work. A good rule of thumb is to use 30–40 characters per line for mobile.