The first video below (2 minutes) shows how to set the reading level in Google as of February 2014. Since Google sometimes changes its user interface, the advice in the video may be out of date. If so, you can find out about the current way to set the Google reading level by going to the following link: Google reading levels
This is an extract from the above page:
How reading levels are determined
The reading level for a particular result is determined by a computer program. When the program was created, thousands of web documents were rated by teachers as basic, intermediate or advanced. These ratings helped the program understand what factors led results to being easier or more difficult to read. When new results appear, the program determines their appropriate reading level based on factors such as vocabulary, word length and grammatical complexity.
Note that the first few words of the video are missing: This video will show you how to find web pages, which are written in easier English.
The following video shows the previous way to access the reading levels: