mysql> select count(*), avg(cast(datediff(hire_date, birth_date)/365 as decimal(20,15))) from employees join dept_emp using (emp_no) join departments using (dept_no) where dept_no='d001' group by dept_emp.dept_no;
+----------+------------------------------------------------------------------+
| count(*) | avg(cast(datediff(hire_date, birth_date)/365 as decimal(20,15))) |
+----------+------------------------------------------------------------------+
|    20211 |                                           31.5440424069036663203 |
+----------+------------------------------------------------------------------+
1 row in set (0.07 sec)

mysql> select count(*), avg(cast(datediff(hire_date, birth_date)/365 as decimal(20,15))) from employees join dept_emp using (emp_no) join departments using (dept_no) group by dept_emp.dept_no;
+----------+------------------------------------------------------------------+
| count(*) | avg(cast(datediff(hire_date, birth_date)/365 as decimal(20,15))) |
+----------+------------------------------------------------------------------+
|    20211 |                                           31.5440424069036663203 |
|    17346 |                                           31.5176717535990430070 |
|    17786 |                                           31.5703832006986956033 |
|    73485 |                                           31.5165073474792270531 |
|    85707 |                                           31.4997869928167594246 |
|    20117 |                                           31.5653590872056966745 |
|    52245 |                                           31.5219184632247296392 |
|    21126 |                                           31.5049994872476569156 |
|    23580 |                                           31.5070884306113231552 |
+----------+------------------------------------------------------------------+
9 rows in set (1.68 sec)