Why can't I call an older co-worker "Grandpa"?

Dear Diversity Diva: In the restaurant where I work, we young people call this older guy we work with “Grandpa.” We don’t mean any harm, but our boss says we need to stop calling him that. If Grandpa doesn’t mind, why do we have to stop? — Age Ain’t Nothing But a Number

Dear Age: I’m sure you and your co-workers mean no disrespect when you call your older comrade Grandpa. It sounds like it may even be a title of affection and admiration.
However, your boss has more to consider than just whether you like calling a co-worker something other than his name or even whether Grandpa minds the nickname.
For one, I get the impression that you’re assuming Grandpa doesn’t mind being called by that name. You may be right. However, I don’t know if your older co-worker is 35 or 75. Because employees over the age of 40 are legally considered to be in a protected class— meaning that they can sue on the basis of age discrimination — your supervisor may be concerned that Grandpa might claim age discrimination someday and cite your playful nickname as part of the complaint.
Also, remember that your boss has to take into account that other older co-workers and customers who overhear in the restaurant may take offense.
Another possibility you may not have considered is that Grandpa really doesn’t like that nickname and he is the one who went to the boss and complained about it. Many times, even the friendliest co-worker feels more comfortable going to the boss on the sly rather than taking complaints directly to the source for fear of confrontation or creating tension. Even the most direct person has been known to take that approach to keep calm waters and to make the boss the heavy.
A lot of times, conflict in the workplace comes from the impressions created by workers who mean no harm. In a case like this, therefore, the best thing to do is just follow your boss’s guidance. Call Grandpa by his given name.