It depends on the agreement. Generally an employer would not break a severance agreement by filling your old position because severance agreements are usually designed to state what you employer must pay to you as your final pay package. Depending on the circumstances you may have an age discrimination claim, but you would need to consult an attorney to review your agreement and the facts surrounding the layoff.
Most severance agreements advise you to consult an attorney before signing. If you did not consult an attorney, you may want to do that now. It is difficult to get out of a signed legal document, especially if it provided a week for you to change your mind (which most severance agreements do) and that time has already passed.
Unfortunately, without seeing what you have signed it will be difficult to provide a useful answer. I am sorry.
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