If you are over 40 and the severance agreement includes your waiver of any age discrimination claim, then the employer should give you 21 days to review the agreement. Otherwise, there is no obligation for the company to provide more time for your to review. In fact, the company can withdraw the offer at any time before it is accepted. Good luck.
My answers to questions posted on AVVO are intended to provide general information only, and are not intended to be legal advice. Employment law issues typically require a careful case-by-case analysis. Consequently, if you feel that you need legal advice, I would encourage you to consult in person with an employment attorney in your area.