In an effort to comply with GDPR, Twitter is blocking users who were underage when they signed up for the service – even if they’re now well over 18.

The company instituted a wave of account suspensions on 25 May, the day the new privacy regulation came into effect, locking the accounts of any user whose self-declared date of birth suggested that they may have been under 13 at the point they signed up for the account.
https://www.theguardian.com/technology/2018/may/31/twitter-blocking-users-who-were-underage-when-they-signed-up