Is it possible to attach sample workbook?
Tableau will only show marks (and thus colors) when there is data. You will have to scaffold your data so you have data for each combination of Category and Age Bucket.
There are many resources available how to scaffold your data source, including this one: Scaffold data with Tableau Prep to fill gaps in your data set | Tableau Software and Data Scaffolding in Tableau
Thanks for helping me Ewald / Pratik. A colleague of mine did give me a solution which worked. Instead of creating Scaffold data, I created a Calculated field with following formula :
zn(lookup(sum([Number of Records]), 0))
I used this in my Worksheet Measure instead of "Number of Records" which gave me what I needed.