Can companies set requirements to withhold servence pay from older workers

Recently my company sold our business contracts and is in the process of shutting down operating units within the plant. One area of the plant will remain running and they have decided to staff that area with employees with the longest employment, no safety or discipline issues and work history. They refuse to allow these long time employees the opportunity to opt out and receive a severance package. The company position is that no one can volunteer for a package. Others in the plant will be able to get a package if they remain until the company is ready to release them. Since this will affect the older employees, some close to retirement age,is this practice legal? These are all hourly employees.