Trump ally and former United States Secretary of Housing and Urban Development Ben Carson has brushed off concerns about the former president's age and claims he is "as sharp as he ever was".

The 77-year-old is expected to face off against Joe Biden in this year's US presidential election as he bids for a second term.

Trump's legal concerns may not be his only concern, with some questioning whether someone of his age should be stepping up to the plate.

But according to Carson, this should not be a worry for US citizens.

"He is as sharp as he ever was", he said.

"He had a gaffe recently but who doesn't do that? It was an isolated incident."