Joe Biden joked that he "looks like he's 40" as he delivered a campaign speech in Philadelphia, where the Kennedy political family endorsed his re-election campaign.
"I know I look like I'm 40 but I've been doing this a long time, but I've never been more optimistic about our future," the president told supporters.
Their backing is a rebuke of Robert F Kennedy Jr’s independent bid for office.
Mr Kennedy's sister, Kerry Kennedy, praised Mr Biden as "a champion for all the rights and freedoms that my father and uncles stood for."