Should I be worried if in a job interview I don't appear excited or even desperate to get a job?

It seems to me like they only hire you if you really really really want that job. But people get older and at some point you have to replace them. It's not like asking old people if they really really really wanna retire, they can't continue a job anymore at some point.

Should I be worried if in a job interview I don't appear excited or even desperate to get a job?
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