It's not that. They're saying that the birthday you entered, combined with the date you made your account, means you were under 13 when you started using Twitte

It's not that. They're saying that the birthday you entered, combined with the date you made your account, means you were under 13 when you started using Twitter. They're known for banning people for this, even when the person is an adult now - I've read about this before.