FAR 61.35-a-2(iii) states: Date of birth, which shows the applicant meets or will meet the age requirements of this part for the certificate sought before the expiration date of the airman knowledge test report.
The minimum age for the ATP is 23 and the test is valid for 2 years. That would make the minimum age to take the test as 21. I know that if you work for a FAR
121 Air Carrier your ATP Written Test never expires. However if you never get the job or you lose the job, your test immediately becomes invalid. Therefore, you cannot use this job as a prerequisite to take the exam.
Link to FAA eCFR Part 61.35