Suzy Reingold, Cushman & Wakefield’s chief operating officer, is suing the Manhattan real-estate giant for age and gender discrimination after it allegedly passed her over for a promotion.

Reingold, 66, claims in court papers that she was denied the job of tri-state president — despite 15 years of management experience — in favor of 38-year-old Ronald Lo Russo, who was hired from outside the company.

The suit alleges that “high-level” company executives called older workers “dinosaurs” and that former CEO Glenn Rufrano said, “We need young people, we need young management, we need young people in this office.”

The complaint, filed Monday in Manhattan Supreme Court, seeks $20 million in back pay, unpaid wages and damages.

“These allegations are completely without merit, and we will defend ourselves vigorously,” C&W said in a statement.

The firm said Lo Russo “has brought incredible leadership to this role” and “a level of collaboration among our professionals who are highly focused on bringing the company to the next level in New York.”

On multiple occasions, C&W CEO of the Americas Jim Underhill “reassured” Reingold that the job of tri-state president “is yours,” according the complaint.

Reingold herself suggested Lo Russo, then a broker with Vornado Realty Trust and chairman of the Young Men’s/ Women’s Real Estate Association, for a mid-level position.

On two separate occasions, the complaint alleges, CEO Underhill’s only question was “How old is he?”

After Lo Russo was hired for the job she wanted, Reingold was promised a promotion to the newly-created position of New York Tri-State Chief Operating Officer, and was to become part of an “Americas” management committee with the title of executive vice president.

The complaint filed by her attorneys, Sanford Heisler, alleges that this never occurred and when she asked to have her title changed it was “denied” by human resources, while she has also been marginalized and excluded from executive decisions.

An attorney, Reingold was hired in 2005 as executive managing director and supervised over 180 brokers and over 190 non-broker staff as head of the Midtown and Downtown.

The suit claims she was paid $100,000 less than two men in Connecticut and New Jersey who each managed half the number of employees.

In 2008, she was advised her then new $300,000 salary would be reduced by 5 percent as part of company-wide cost cutting.

When she complained that violated her contract, she was told to either quit or never bring it up again, according to the complaint.

C&W is the largest private real-estate company in the world with 15,000 employees and 253 offices in 60 countries.