Richard Hewett, 49 when he failed to get on the show in 2005, says Mr Trump and producers have violated the Age Discrimination in Employment Act.
His case claims just two finalists over six series have been older than 40.
"We have had very few people over a certain age apply to be on the show," responded Mr Trump in a statement.
"If they did and we liked them, we would love to cast them on the show."
A spokesman said that a "minuscule percentage" of more than one million applicants to the programme had been over 40.
Mr Hewett's case names as defendants Mr Trump, the Trump Organization, Trump Productions, producer Mark Burnett, Mark Burnett Productions and three affiliated California companies.
The case was filed last week in US District Court.
Mr Hewett, a technology manager from Hampstead, New Hampshire, is seeking unspecified damages.
He told the Associated Press news agency: "People watching [the show] get the impression that if you want to work for a big organisation like the Trump Organization you have to be young.
"The outcome I'm looking for would be to see a recognition on their part that it's in everyone's interest to expand the diversity of the candidates on the show."
The Apprentice sees aspiring entrepreneurs battle to win a place in Mr Trump's company.
The hit UK version is fronted by Amstrad boss Sir Alan Sugar.
Article from BBC News