Tesla CEO Elon Musk responded to criticism of President Joe Biden‘s age and fitness with a cheeky remark during a conversation on social platform X. What Happened: On Thursday, DogeDesigner, a pseudonymous user who works as a graphic designer in the Dogecoin (CRYPTO: DOGE) ecosystem, raised questions about the incumbent’s public interactions, saying, “Every time I see Biden speaking, I wonder how this man became the President of America.” Musk, who frequently interacts with DogeDesigner on X, responded by taking a dig at Biden. “Late-stage civilization vibes,” the tech czar said with a grimaci…