Legal analysis: it is ok not to release people during the service period. Many posts with this age limit are in compliance with the law and can only resign after the expiration.

Legal basis: Civil Code of People's Republic of China (PRC).

Article 577 Where a party fails to perform its contractual obligations or fails to perform its contractual obligations in conformity with the contract, it shall be liable for breach of contract such as continuing to perform, taking remedial measures or compensating for losses.

Article 578 Where a party expressly expresses or shows by his own behavior that he will not perform his contractual obligations, the other party may require him to bear the liability for breach of contract before the time limit for performance expires.