If I decline to sign a separation and release agreement within the 21 day period. Is it null and void?

No, it is not null and void.  Federal law requires employers to give an employee who is 40 or older at least 21 days to consider a separation agreement that contains a release of claims under the Age Discrimination in Employment Act.  During the 21 day period, the employer may not retract or modify the agreement.  If the employer does modify the agreement, a new 21 day period begins.  If the employee has not accepted the agreement within the 21 day period, the employer is allowed to withdraw the separation agreement and release. However, the employer could still allow the employee to sign the agreement after the 21 day period if it chooses to do so. The clause you refer to is common in employment separation and release agreements.  It is possible that your employer would consider removing this language.  However, you would need to talk to someone at the company to discuss.