That sounds an awful lot like age discrimination, unless your job is something that specifically needs to be done even when the place is closed (and no one else's is). An employer is not allowed to treat an employee different because of his or her age. There aren't any specific laws that would require paying employees for days they do not work, or providing PTO, but an employer who provides PTO or paid holidays must do so fairly to all employees across the board. They don't have to pay you for the days the plant is shut, but they cannot force you to use vacation days when no one else is required to do so. They can ALLOW you to take vacation days to avoid losing pay, but it's your decision.
The only exception, really, would be if they have a policy where unused vacation days are forfeited at the end of the year (again, applied to everyone). In that scenario, it would actually be to your benefit to use the days rather than watch them vanish. That could explain why this isn't an issue with short time employees - because they haven't been there long enough to accrue much vacation time. You may want to check your employee handbook, just to be sure.
Otherwise, talk to your HR department. If you are a member of a union, try talking to your union rep. If neither of those things works, it may help to have a local attorney contact your employer on your behalf.
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