Hi, I think first you need to create the bucketing variable in the Data & Analysis tab (field editor, bucketing).. If Age is coded as a number, ranges will be easy to set. If it's a set of text values, you'll need to drag & drop the entries into the range buckets.
Once you create that variable, you can use it in dashboards.

Number: Displays the number of responses in the selected fields (i.e. the number of respondents choosing a value on a five-point scale).

Select the number in the widget and set it as a percentage in the option.

If you have a number set in your dashboard data this will work.