May be a query like below would do the job.
SELECT
CASE WHEN (DATE_FORMAT(NOW(), '%Y') - DATE_FORMAT(birth_date, '%Y') - (DATE_FORMAT(NOW(), '00-%m-%d') < DATE_FORMAT(birth_date, '00-%m-%d'))) <= 20 THEN '1-20'
WHEN (DATE_FORMAT(NOW(), '%Y') - DATE_FORMAT(birth_date, '%Y') - (DATE_FORMAT(NOW(), '00-%m-%d') < DATE_FORMAT(birth_date, '00-%m-%d'))) <= 30 THEN '20-30'
WHEN (DATE_FORMAT(NOW(), '%Y') - DATE_FORMAT(birth_date, '%Y') - (DATE_FORMAT(NOW(), '00-%m-%d') < DATE_FORMAT(birth_date, '00-%m-%d'))) <= 50 THEN '30-50'
WHEN (DATE_FORMAT(NOW(), '%Y') - DATE_FORMAT(birth_date, '%Y') - (DATE_FORMAT(NOW(), '00-%m-%d') < DATE_FORMAT(birth_date, '00-%m-%d'))) <= 50 THEN '50-100' END AS age,
COUNT(*) total
FROM dob_table
GROUP BY age;
WORKING DEMO
Input:
| id | birth_date |
|----|----------------------------|
| 1 | February, 01 2014 00:00:00 |
| 2 | February, 01 2014 00:00:00 |
| 3 | February, 01 2014 00:00:00 |
| 4 | February, 01 2010 00:00:00 |
| 5 | February, 27 1989 00:00:00 |
| 6 | February, 27 1989 00:00:00 |
| 7 | February, 27 1989 00:00:00 |
| 8 | February, 27 1989 00:00:00 |
| 9 | February, 27 1989 00:00:00 |
Output:
age total
1-20 4
20-30 5