Mayoral candidate Anthony Weiner called an older political rival "Grandpa" during a heated confrontation at a forum sponsored by the AARP.
When McDonald disagreed, Weiner said, "Yes, you do, Grandpa."
The exchange was caught on camera by NY1. In a statement, the AARP described "some comments" made at the forum as "unfortunate."
"A person’s age should not be a factor in politics, or anything else," said Beth Finkel, state director of the New York AARP.
The organization said that New Yorkers over the age of 50 will likely decide the outcome of the mayoral race, adding that one of every two voters in the November election is expected to be an AARP member.
Weiner's campaign couldn't immediately be reached for comment.
McDonald, who founded an organization that helps the homeless find work, was booed by the crowd when he taunted Weiner as a "glib narcissist."
McDonald and Weiner are struggling in their primaries.
A poll of likely Democratic voters released late last month showed Weiner in a statistical tie for second place, with 16 percent compared with 27 percent for City Council Speaker Christine Quinn, 21 percent for Public Advocate Bill de Blasio and 20 percent for former comptroller Bill Thompson.
Among likely Republican primary voters, 7 percent supported McDonald. Former MTA chairman Joe Lhota came in first place with 49 percent, followed by billionaire John Catsimatidis at 35 percent. The Quinnipiac poll questioned 446 likely voters and had a 5 percentage point margin of error.
Several of Weiner's rival candidates have questioned his trustworthiness and called for him to leave the race. His campaign manager has also resigned.
Weiner has said he will stay in the race.