t_dept_avg_tab
is a SQL type so you can use it directly in a FROM with a TABLE() function:
SQL> select dept, avg(age)
2 from table(t_dept_avg_tab(t_dept_avg_rec(20,60)
3 , t_dept_avg_rec(30,40)
4 , t_dept_avg_rec(20,24)
5 , t_dept_avg_rec(30,66)))
6 group by dept
7 /
DEPT AVG(AGE)
---------- ----------
30 53
20 42
SQL>