How many respondents younger than 25 years old did participate in the survey in 2016?
respondents' and 'users' are synonyms; younger than 25 years old refers to AnswerText(SurveyID = 2016& QuestionID = 1)< 25
SELECT COUNT(DISTINCT T1.UserID) FROM Answer AS T1 INNER JOIN Question AS T2 ON T1.QuestionID = T2.questionid WHERE T1.QuestionID = 1 AND T1.SurveyID = 2016 AND T1.AnswerText <= 25