But it is ignored every time. But now it is not the time as if you have registered before you completed 13, your account will be banned on a permanent note. The new privacy laws et up by GDPR (General Data Protection Regulation) has forced Twitter to take everything seriously and the users are banned without second thoughts.
The DOB will be verified with a valid proof and be failing to provide will result in a permanent ban. If you are a regular user, you might have been welcomed by this message, "In order to create a Twitter account, you must be at least 13 years old. You don’t meet these age requirements."