Sentence:  We have attempted to simplify the extensive task of analyzing <WSD>onset</WSD> ages and completion ages of each child - more than 1700 values for the entire group - by constructing figures for each of the 21 centers so that the data for all 34 boys and 34 of the girls will appear together for each growth center . 
Meaning of the word marked with <WSD> tags:  the beginning or early stages