Can not group ages in pivot table after creating relationships between tables.

[INN]

Hi
I have 3 tables (please see the link below). I created relationships between these 3 tables. The main table has Employee info but missing age column. Age column is in another table. When I tried to create pivot table (Rows = Age, Values = Salary) but when I tried to group Age, I got an error message (can not group this selection). Why is that? Thank you very much.


Loading Google Sheets