You may use Issue dimension Epic hierarchy in rows (Epic level issues) to get all epics listed in the report. Then add, in Pages, dimension “Issue type” and select “Sub-task” (or, if you have several sub-task types, select all of them). Now, you have epics in rows, but values would be calculated only from their sub-tasks.
Average age
Use standard measure “Average age days” (Measures > Calculated members).
Of sub-tasks with specific values in CustomFields
Add, in Pages, “Client” dimension and select “ValueA” to get all measures calculated from sub-tasks (as you already have sub-task types selected in Pages) with ValueA as a client.
In all issues of given Epic
When Issue Epic dimension is selected in rows, measure “Issues created” gives the total number of issues per epic and accordingly to the Page selection.
When “Issue type” = Sub-task is selected in Pages, this measure returns a number of sub-tasks per epic; if also “Client” = ValueA selected in Pages, the measure shows the number of subtasks with client ValueA for each epic.