WATCH: Reporter asks Trump 'how old is too old' to be president. Trump's hilarious response takes shot at Biden.

President Donald Trump had a hilarious response for a reporter who asked Friday how "old is too old" to be president.

"How old is too old to be president?" the reporter asked Trump outside the White House.

Trump's response took a shot a former Vice President Joe Biden, the frontrunner for the 2020 Democratic presidential nomination.

"Well I think that I just feel like a young man. I'm so young, I can't believe it! I'm the youngest person— I am a young, vibrant man," Trump said with a sly smirk. "I look at Joe, I don't know about him."

The reporter's question comes amid concern over the leading candidates for president in 2020. Trump is 72 years old while the leading Democratic candidates, Biden and Sen. Bernie Sanders (I-Vt.), are 76 and 77, respectively.

Dr. David Scheiner, who previously worked as Barack Obama's personal doctor, expressed concern over the elder candidates in a recent interview with the Washington Examiner.

"I think we're asking for trouble. We don't know what their health is. They look spry but I don't think that's enough. You can't accept it at face value," he said.

(H/T: Liz Wheeler)