In 1990, Congress passed the Older Workers Benefit Protection Act (OWBPA) which amended the Age Discrimination in Employment Act (ADEA) to safeguard older workers' employee benefits from age discrimination.
If you are at least 40 years old, a federal age discrimination law—the Older Workers Benefit Protection Act (OWBPA)—requires your employer to give you at least 21 days to consider the severance package. (You are entitled to 45 days if the deal is part of an early retirement incentive program.) If your employer doesn't give you enough time, your waiver of any age discrimination claim is not valid, and you could still sue your employer.
Your employer can't withdraw its offer of severance during the waiting period. And, you don't have to wait the full 21 (or 45) days; you can sign the agreement earlier, if you wish.
Under the OWBPA, your employer must also give you seven days after signing to change your mind and "revoke" your acceptance of the severance offer. This seven-day period is required by law; neither you nor your employer can waive it. Because of this, you probably won't receive your severance check until at least seven days after you accept the deal.