Basically, you have no leg to stand on.  Unless you have an employment/union contract or there is company policy to the contrary, you must work the hours that you have been scheduled for.  In an "at will" employment relationship (and most are), an employer cannot only hire/fire employees as it wants (with or without reason), it can also increase/decreasesalary/hours, promote/demote, and generally impose requirements as it sees fit.  In turn, an employee can choose to work for their employer or not.  The only exception would be if discrimination played a role in the situation; age can place a person in a proteced class under workplace laws.  However, under the facts presented here, age per se does not appear to be such a factor.  Bottom line, if you refuse to work your scheduled shift, you could be terminated for cause.