When are you too old to be President of the United States?
The answer, according to Donald Trump, is not when you are in your 70s.
The current US President, who is 72, could face competition from 76-year-old Joe Biden who on Thursday announced his bid to be the Democrats' presidential candidate for the 2020 election.
At 77-years-old, Bernie Sanders is the oldest candidate in the running and has also launched his Democratic bid.
Asked by a reporter, how old is too old to be President of the United States, Trump replied: “I just feel like a young man.
"I’m so young.
"I can’t believe it.
"I’m a young vibrant man.”
Then he smiled and said he was not sure about Democratic presidential contender Mr Biden.
“I look at Joe. I don’t know about him," Mr Trump said.
But he added that he would never say anyone is too old to be President.
It is not the first time this week that Trump has mocked Mr Biden, who was Barack Obama's vice-president.
Shortly after Mr Biden announced he would be launching a third run for the presidency, after two unsuccessful attempts at earning the Democratic nomination in 1988 and 2008, Trump tweeted: "Welcome to the race Sleepy Joe."
Mr Trump also suggested Mr Biden could not win the Democratic nomination stating: “I only hope you have the intelligence, long in doubt, to wage a successful primary campaign."
Mr Biden made his announcement to seek the presidency in a video posted on Thursday.
In doing so he has become the most noted of Democratic candidates competing to take on Trump in the 2020 election.
In a video posted on Twitter he said: “We are in the battle for the soul of this nation.
"If we give Donald Trump eight years in the White House, he will forever and fundamentally alter the character of this nation, and I cannot stand by and watch that happen.”
He continued: “The core values of this nation, our standing in the world, our very democracy – everything that has made America America – is at stake.
"That’s why today I’m announcing my candidacy for president of the United States.”