New to Data Modelling and Power BI - Basic 3 Question Survey Data & Reporting Issue

You are trying to use a fact (age range response) as a dimension (the age slicer).  That won't work as by doing that you are filtering away all other facts.  Rethink your data model, normalize the age response into its own table.

Ages = calculatetable(distinct(DimAnswers[response_value]),DimAnswers[question_no]=2)

use that table to feed the slicer

Create another table that joins the respondent IDs to the ages

Ageresponses = SUMMARIZE(filter(Responses,[question_no]=2),[id],[response_value])

Wire them in the data model

And you get your desired result. See attached.